Hope someone has seen this before, and can give me some ideas. 99 F150, 4X4, 4.6, Auto. Runs fine when the outside air temp is below 70 degrees. Problems occur when the weather heats up. The truck will not restart after being driven in the heat and turned off. It will crank and initially stumble, but will not start. If it sits for 45 min. to an hour, it will start right up and run fine. When it gets extremely hot, over 100 degrees, it will stall while driving and then not restart until it has sat for the aforementioned period of time. It has been to the dealer twice, but each time the problem could not be duplicated. Changed fuel filter, both coil packs, plugs and wires. The last time it happened, I pulled a plug out and got good spark while the truck would not start, so it appears to be a fuel problem. Could the fuel pump be heating up and shutting down temporarily? Would a PCM get hot and shut off the fuel pump? Is there any way the truck could be vapor locking? Any help will be appreciated.

Thanks!