Your gas cap might not be venting which creates suction in the tank - the longer you run it the worse it gets and letting it sit a while often relieves it for a bit.  Also, potentially, the filter before your fuel pump might be creating "inadequate suction pressure head" - in other words too much restriction in the low pressure side of the pump.

What kind of spacer are you running between your carb and manifold?  Overheated bowls in the summer can create weird symtoms with the needles and seats - sometimes gas will blow past and other times closing them off.  What is the exact pump you are running and what size is your fuel line from the tank?  Any chance the small flex hose from the tank is kinked and restriction flow?

I guess another possiblitiy could be a clogged pickup tube screen in the tank.  Do you know the history of the tank as in new/old/resealed?

When you drain the tank, drain it through a fine white hankerchief or something so you can inspect what is coming out.  Your problem sounds like classic rotten gas tank, but if only 6 years old, and in continuous use, that is not likely.  If it ever sat long enough for the gas to turn to varnish, you might want to spend the $99 and replace the tank again.  What about the steel line?  Is it possibly dirty and scaling off?

Fuel pumps are pretty cheap for your car, under $20, so that might be something to try.

Also check your rubber fuel hoses. They don't hold up to new crappy gas. Take air compressor hose with blower nozzle, then blow from the frame line back into tank. You can also do this with the line unhooked from your sender.  Sometimes crap and rust builds up in the fuel lines and causes funky problems.