Truck Repair: Chevy truck, side exhaust manifold, vacuum switch

Question
Hi,
You gave some excellent suggestions to me before, so here goes again, I have a 1979 Silverado truck, 305 engine, that is driving everyone nuts.  We have changed ALL of the fuel system, tank, all filters,lines, fuel pump several times, carbarator (2 barrel) several times, have rerouted the fuel system to eliminate one tank and the crossover valve, it now goes directly from the fuel tank to the pump.  This started 15 mo. ago, has been the same through the old carbarator, 2 rebuilt ones (first one was bad) 3 fuel pumps (old pump, one replaced then got bad gas so had to start over cleaning, then last new pump)  Rebuilt engine (for other reasons)Then I discovered a heat/time relationship, so we were told it might be the coil, changed the ignition coil, didn't fix it.  When you drive it over 55 for almost exactly 1 minute when the heat is 90 or better, it starts jumping, sputtering and sometimes dies, acts/feels like fuel, but time doesn't make sense.  Will do it every time.  Will also do it in the driveway, if you rev the engine up high and hold it for a minute.  If it's cooler it takes longer (2 min) but still does it.  What could be getting too hot and cutting out?  HELP

Answer
I can think of two things that could cause that.
One is the gas cap not letting air into the tank, and a vacuum forming, thus starving the system.
The other is the heat riser valve in the right side exhaust manifold not opening, and overheating the intake manifold, since when that valve is closed, the exhaust from the right bank passes through a port in the intake manifold to the left side, and out there.
There is a vacuum hose to that valve, which passes through a temperature vacuum switch, which I have had problems with.
If you disconnect the vacuum hose to the valve in the exhaust pipe to manifold connection, the valve opens by spring pressure.Actually, a plugged exhaust system is a third possibility, but I bet on the heat riser.
If you think it might just be running out of gas volume, you could stick a "T" near the carb, and connect a pressure tester, and when the problem occurs, see if there is fuel pressure. Ideally, have a pressure tester with a bleed valve, and see if there is a buildup of vapor in the line.
Low volume could be a kink in a metal line along the frame, or some other restriction.
If there appears to be vapor, catch some fuel in a can and see if it is hot, or if the pump is sucking air, like through a crack in a rubber hose.
Van