I have a 1990 S10 4X4 TAHOE EDITION 4.3 V6....It runs great until it warms up to operating temperature....Once it's warmed up it runs like crap. It's like it doesn't want to go unless I give it a good amount of gas and then she goes, Once I level off the throttle it runs crappy again, sometimes chugs, etc. until I give it some gas again. I have noticed that the temp gauge never moves up anywhere near the approx 190 mark or so, always down way low on the gauge.....Could it be a bad or stuck open thermostat and would THIS be a problem that would cause it to run like this. Thats what one friend seems to think and another said he thought it was the EGR Valve....I'm miffed....Any help would be greatly appreciated.

 

Bad thermostat will cause big swings in temperature. Mine stuck open, truck would never warm up, when it finally did, it ran hot.
I blocked off my EGR years ago, can't remember what it ran like, but it was annoying.
It could be a bad O2 sensor. When the motor is cold it runs in cold loop, when temp is reached it switches to O2.

 

Could be alot of things, but the first step is to check for codes. Then start heading in that direction as far as diagnosing, but don't just start replacing parts without finding the exact problem.