1991 Chevy Silverado does run right when warm

You don't say what you've done so far, but I had a similar problem that I just solved with a 02 v6 Camaro.
Replace fuel filter if you haven't. Replace spark plugs and wires if old. My problem was a clogged cat converter, which caused my engine to run hotter than normal, and my ignition coils gave out. Ran fine when cold, horrible when hot, especially with A/C on and passengers and rpm around 1500. Coils tested fine with ohm meter but that doesn't mean they are still good. Resistance rises when temp goes up.

My car wasn't overheating, but I did notice that it was running slightly hotter than normal, coolant temp
a little higher, oil pressure a little lower, indicates that engine is running hotter than normal, but not overheating.