Won't start on 100 degree days.

[Red 87 Chevy]

I asked this question on other 73-87 BB's but the problem continues. I have a 350 TBI. On the very hot days, restarting the engine is difficult if sufficuient time to cool had not passed. For example, I drive the truck about 10 miles, turn off the motor for about 30 minutes, and then is just doesn't want to start. This almost always happens on the 100 degree plus days. The fuel pump and filter are new. Replaced the plugs about 200 miles ago. Sufficient fuel pressure. Volume of gas in the tank does not matter. TBI is clean and working properly. All valve cover vents and hoses are clear. Timimg on on the mark, even when the engine and weather is hot. After the motor cools for over 1 hr, or the outside temp drops into the 80's, the engine runs perfectly.

The truck's been to the Chevy garage. They find no problem, but experienced the problem with my truck today. They are stumped, too.

Any suggestions on what is causing this problem?