2001 Buick Regal Dies in Hot Weather

For 3 years and through 3 mechanics, I have had this problem. The problem occurs only when the temperature outside gets above 90 degrees, and I do some neighborhood “errand driving” (go a couple of miles to the post office, stop, start and go to grocery, stop). After a while, I will be driving at speed and the car will die. Everything all at once. Gauges drop, electronics die, power steering, etc., die and engine stalls right there. If I try to turn the key, there is no sound. I have to open the hood and let the car sit for 20 or 30 minutes, and then I’ll be able to drive it again, but only for a limited time berfoe it happens again. Next cool day, and it’s fine. Buick dealers offered no help because they could not “replicate the problem.” Indie mechanic wants to replace PCM ($700 parts & labor) because he DID replicate the proble but said there were no diagnostic codes to help.

Question is - Will replacing computer CURE or just give us insight to what the problem is?

Thanks much!