I had this same problem with mine, aluminum heads + aluminum intake. The only thing that stopped it was Loctite.

Mark some bolts to confirm that they are actually turning.  You can also get this if you are somehow distorting the intake rather than just torqueing bolts.  If you used end seals and they are too tall, for example, or if the intake is too narrow after machining or due to the heads or if someone trimmed the intake you will be springing the intake as you tighten and the bolts will be more likely to back out through heat cycles. Since many people use aluminum heads and/or intakes and don't have to constantly re-torque there may be some other problem you are fighting. I fought this for years on an FE and it turned out to be an aftermarket head where the intake surface was out of square - Dove quality control.  You may need to look at thicker gaskets and/or verify that all parts are square and true and in spec.  If the heads are after market, contact the manufacturer for torque values as the suggested torque will depend on material and if the threads are coiled or not on aluminum heads.

Follow my instructions on the web page.  I had my 351W aluminum intake on Edelbrock aluminum heads for over 15 years and never had to retorque one time.  And I used stainless steel ARP bolts.

I found this issue was another reason to use RTV in place of the end gaskets.  As MS posted (although I have just 7 years), I have aluminum intake w/aluminum heads, ARP SS bolts and no problems.
PS - the torque on these bolts is not that great, I do not believe this is a bolt "stretch" issue ... unless you're using low grade bolts.

Lance, the torque for the standard FoMoCo intake/head combo is 23-25 ft. lbs. with the stock cap screws (bolts).  I think ARP will have a torque recommendation for using their products, which may be different from Ford's.  Also, I don't think LocTite is recommended for SS fasteners.
Enjoy

Maybe I should clarify something on torquing the intake.  I do this every time I install one and never have to go back, but maybe I am not making this part clear, or even known.

Anyone who has ever torqued an intake knows how tightening one bolt will make the next one be very loose.  When I am torquing one down, I follow the circular pattern working from center bolts outward, but I repeat that torque procedure about 4 or 5 times in a row or until all of the bolts click the torque wrench with about the same amount of movement of the wrench.  Once they all feel the same, then they are all actually torqued.

If you torque one to the desired setting, then torque the others without repeating the process, the first ones you did are going to be very, very loose.  If you are using any more than a paper thin coating of RTV on the gaskets, you will probably never get a properly torqued intake manifold.  And it is always a good idea to have the gaskets held to the heads with contact cement so they cannot move.  Contact cement is applied to both mating surfaces in a very thin layer, not even paper thin.  Let both dry (that is the step most omit and it backfires on them every time) and then once dry, place them together and they will never move on you while installing the intake.  If you install contact cement wet, you are wasting time and contact cement and the gaskets will slide on you.