engine goes flat on long hot upgrade; looses power. Can correct by turning off 60 seconds and restarting

What is your question?

In drive gear on a long up grade the power will at first falter, slow down and then no matter to how much fuel you send to the engine it just slows the car to a crawl. If we pull over, turn off the ignition, restart after 60 seconds we are good to go but problem will reoccur if still on long up grade. On the other hand, if I place the tranny into third gear, we do not seem to have the issue going up hil.

What makes this problem better or worse?

It's worse when engine is hot

How long has your 1996 GMC Yukon had this problem?

A few years; mechanics have been unable to diagnose