I have a '94 Sierra with a 350. After it runs for a while, it gets where it doesn't want to run without me keeping my foot on the gas a bit, and if it does die, it won't turn over for 15-20 minutes. It gets worse when it's raining. I have no idea why this is happening, or where to start looking.

 

Time to clean engine grounds. Corroded high resistance.

 

If that's the ground cable between the battery and engine, it's clean and tight.

 

I had the same issue with my ol 97 Sierra.
I lived with it for almost a year until my fuel pump crapped out.
After a new fuel pump I never had the problem again

 

Ones at thermostat housing also?

 

Good point... The ground at the thermostat is your engine wiring harness ground.

 

I'll check that. Thanks.