96 jimmy runs great at first then when the temp gos up under the hood starts to lack power thjen evenually dies....20 minutes later starts and will drive again and engine does get hotter then 170 degrees F

I was having a similar problem and where the more gas I gave the slower it would go, sometimes even backfiring. If I threw in neutral and restarted it was fine for 2 mins again. It was the comshaft positioning sensor wich is under the rotor. remove the cap and you will see the wires, the black part inder the metal circle deal.

Check for water in oil on dipstick. head gasket? head cracked? need more diagnostics and info.