Electrical Heat Problem

From : old traveler

Q: huuuuum i guess it could be. i have been driving this vehicle for years. why would it start now for no apparent reason also i can run it at interstate speeds without any falter which indicates to me that it is getting plenty of fuel. ot yea..it sounds like a non electrical issue called vapor lock.... i have a 1985 318cu engine that is wearing me out. this may be a little hard to explain but here goes. the engine appears to run well until it has been driven for about 20-30 minutes in 90 degree plus heat. even then it does ok until i stop for a minute or two like at a stop light. when i attempt to start moving the engine misses and falters. if i really feather the accelerator i just can get it going. once it has picked up a little speed it will run ok under constant speed. i have changed the ignition module the ignition resistor the battery and the alternator. i have found that if i cut the ac off i can get away much better. now on hot days when i have to stop i will cut the ac off until i get rolling again. it appears that the problem is electrical/vacuum related. i am thinking that maybe the distributor pickup may breaking down under heat. the problem will reoccur at road speed if i floorboard it. then it will start breaking down. this is what makes me think the problem is vacuum advance related. any one have and thoughts ot .