Car stalls

I recently bought a 1994 Taurus wagon (3.8 litre engine, 125 K miles) and it also stalls. The conditions when it stalls are when it is very hot and I have driven the car some ways. When the weather is below 80 it is fine, but above 90 and it also stalls. I spent over $400 "fixing" the exhaust (which sounded fine) but this did not change anything. My garage says that it will likely cost more to diagnose the problem than to fix it. Does your car stall in the heat, or just any time. My garage says it might be the gas cap, but the Ford dealer says it is likely that the fuel pump over heats (but as it is submerged in cool gas, this does not ring true with me, and it will cost $500 to replace the fuel pump). Would love to find out what is wrong with my car, which is in fantastic shape otherwise

I have actually noticed that in the summer its been doing it alot more and in the winter it doesn't really do it so maybe you are correct. I heard it could be the gas cap too but I don't know. I notice that when I put different gas in (87) it works alot better but gas prices SUCK right now so I been using 85. But I honestly think its the heat its pretty hot here where I live. I try and drive it in the morning or night time so it doesn't just stall out while driving. If you do find out what is up with yours you should let me know I am curious to know