First of all, welcome to the best Mustang forum on the web.  I think you'll be glad you came. 

When this occurs do you loose all the lights or just the headlights?  There is a wire from the headlight switch...yellow #460, I think...that puts horn power to the horn button.  This little dear is known for shorting to ground, especially with aftermarket steering wheels.  This causes the circuit breaker in the HL switch to cycle.  Something to think about.
But I gotta say, True74's suggestion makes sense too.  Crappy primary connections will cause high resistance which reduces the voltage and causes current to go up...which causes heat and could be causing that CB to cycle.
I had this happen on the way to the Bash this year when my brandy new (not stock) HVAC switch burned up due to a loose female spade connector.  That caused 300 miles across Texas with no AC...Any guesses about how Momma felt about that?

BB

 

Welcome!
I would strongly recommend you run relays to your headlights and have the stock light switch  wire control the on/off of the lights. IF you have  new(er) seal beam headlights they are probably drawing more power thru a very inadequate headlight  wiring circuit. The headlight switch WILL tend to cycle on/off when it starts getting warm from a lot of power going thru the old wires.
Just thought you should know.
6sally6

Welcome!  Did you change the headlights out, by any chance?  It sounds very much like the circuit breaker in the headlight switch is cutting in and out.  Often this is caused by the owner putting in higher wattage headlamp units. 

If this is the case you can either go back to standard wattage lamps or, as others suggest, install relays.

There is a good possibility it is your dimmer switch.  step on it a few times and see if it makes a difference.  Mine did that one day after I drove it at night and used the switch.  I clicked it several times and it works fine!

Most likely the issue is that there are no relays in the circuits for the high and low beams.  I had this exact same issue with my '67.  I kept thinking it was the headlight switch, but I was using Ford switches and had gone through a couple with very little mileage on the car.  The new switch fixed the issue for a little while, but it came right back.  After considerable searching I came to understand that modern headlight bulbs draw considerably more amperage than the bulbs these cars had from the factory.  The headlight switch has an internal circuit breaker that gets tripped when the amperage gets too high as a result of heat increasing the resistance in the circuit.  It trips, the lights go off, it cools down a little, the lights come back on.  The permanent solution is to install relays to move the high amp load out of the portion of the circuit with the switch.  This way the switch only turns the relays on and off with a very low amp load, preserving the switch and ensuring the the lights stay on. 

Here is a neat solution to install relays without cutting into the factory wiring harness or drilling additional holes to mount them.  I am running 90/100 watt Cibie headlights.

zakdaddy99 wrote:

I installed a relay/harness kit from Speedway Motors in my 66. I have halogen sealed beams. Before the relays I had around 10 volts at the headlight at idle. I now have the full 13 volts. Brighter headlights and no worries about the headlight switch. It plugs in just like the one Boom posted. No cutting required. Very easy installation. Everything needed comes in the kit. 

Well how about a link for all the folks.  I had to design and construct my own, not realizing that there were ready made setups for it.
 

TKOPerformance wrote:

Really, the ready made setups (which I also used, not knocking them), are a lot of money for two relays and some wire.  It isn't like they're plug and play.  Pretty simply you connect battery power to the relay, power out from the relay to the high or low beams, ground the relay, and connect the wire coming from the headlight switch for high and low beam to the trigger side of the relay.  I spliced into the small headlight harnesses in my '67, so instead of having to change out a whole $300 harness if I were to ever go back to stock (not likely) I could just change out two $15 harnesses. 

It's not that easy if you don't understand wiring!  Wiring is like Algebra either you get it or you don't!!  Some people cant look at a wiring diagram and just bam!!!  there it is.  If you don't understand the basics of wiring how can you know the difference between normally open and normally closed when referring to a relay.  I still have to scratch my head thinking about wiring up a relay.  Have to understand what each terminal does and how to wire it for your application.  With the ready made harness they probably already have the relays wired with color coded wires and all you have to do is connect the green wire on the harness with the green wire that goes to you headlights and so on.
I grew up with a dad who was an electrician and specialized in electronics,  I learned a lot from him and have enough knowledge and skill to rewire my entire house from weatherhead to the receptacles.  HOWEVER,  Automotive wiring, at times makes me say HMMMMM?????  I rewired my headlights to work with relays and I rewired my 65 using an under hood power distribution box from a 04 mustang which contains all the fuses and relays for the car.  It was no easy task but it wasn't brain surgery either.  NOW, there have been a few discussions on here about using resistors to accomplish various tasks in wiring,  there I'm lost!!

Me too Terry!!!!!!
I still can't figger out how to read an ohm meter(or whatever its called.) I can do "primitive"stuff with a testicle light....I mean a teste light!!!! Anyway.... If a Mustang was 120 volt instead of 12....I'd be dead by now!!
It just ain't that simple for some of us.
6s6

TKOPerformance wrote:

Really, the ready made setups (which I also used, not knocking them), are a lot of money for two relays and some wire.  It isn't like they're plug and play.  Pretty simply you connect battery power to the relay, power out from the relay to the high or low beams, ground the relay, and connect the wire coming from the headlight switch for high and low beam to the trigger side of the relay.  I spliced into the small headlight harnesses in my '67, so instead of having to change out a whole $300 harness if I were to ever go back to stock (not likely) I could just change out two $15 harnesses. 

 
Actually, there are some kits that are plug-and-play. You unplug the headlight connectors and plug them into the new harness. Then you plug the headlight connectors on the new harness onto the headlights and run a hot wire to the battery. A fuse and the two relays are included in the new harness. I have seen examples of this kind of harness for less than I could buy the relays and wire. Check eBay if you are interested.

terry wrote:

TKOPerformance wrote:

Really, the ready made setups (which I also used, not knocking them), are a lot of money for two relays and some wire.  It isn't like they're plug and play.  Pretty simply you connect battery power to the relay, power out from the relay to the high or low beams, ground the relay, and connect the wire coming from the headlight switch for high and low beam to the trigger side of the relay.  I spliced into the small headlight harnesses in my '67, so instead of having to change out a whole $300 harness if I were to ever go back to stock (not likely) I could just change out two $15 harnesses. 

It's not that easy if you don't understand wiring!  Wiring is like Algebra either you get it or you don't!!  Some people cant look at a wiring diagram and just bam!!!  there it is.  If you don't understand the basics of wiring how can you know the difference between normally open and normally closed when referring to a relay.  I still have to scratch my head thinking about wiring up a relay.  Have to understand what each terminal does and how to wire it for your application.  With the ready made harness they probably already have the relays wired with color coded wires and all you have to do is connect the green wire on the harness with the green wire that goes to you headlights and so on.
I grew up with a dad who was an electrician and specialized in electronics,  I learned a lot from him and have enough knowledge and skill to rewire my entire house from weatherhead to the receptacles.  HOWEVER,  Automotive wiring, at times makes me say HMMMMM?????  I rewired my headlights to work with relays and I rewired my 65 using an under hood power distribution box from a 04 mustang which contains all the fuses and relays for the car.  It was no easy task but it wasn't brain surgery either.  NOW, there have been a few discussions on here about using resistors to accomplish various tasks in wiring,  there I'm lost!!

For those of you who are electrically challenged,  look into something called Snap Circuits. its a way to teach electricity to kids.  Its really cool and really helps.  its a kids toy, but it will teach you alot real fast.

MS used to have the diagrams...I think JameW did them...on his website.  Very good drawings and simple ckts.  I've drawn and sent a diags to a number of folks...don't know if they do any good butt.......
Between Winsle, JamesW, Ultrastang, BobE, and myself and others there will always be someone that is willing to help with simple ckt drawings.  I don't know how many times I've PM'd my phone #,  but I've had few calls.

My advice for anyone in or getting into this hobby.....DON'T BE AFRAID TO TRY.  SOME IDIOT PUT IT TOGETHER, YOU CAN PROBABLY FIGURE IT OUT IF YOU TRY.

BB