My 91 k1500 w/350 was doing the same thing. I suspected the aftermarket radio or bad power/ground wires but turned out to be the heated (3 wire) 02 sensor that I had wired to a constant 12v source instead of switched 12v. Good luck!
Will these tall valve covers work on a 91 350 tbi? Not sure if short vs tall make a difference?
https://www.ebay.com/itm/Black-Tall-Finned-Aluminum-Valve-Covers-For-Small-Block-Chevy-350-Vortec-TBI-/125076961129?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l49286&mkrid=711-127632-2357-0
Looks like a beast...I like it.
Would bet truck was used harder than it was maintained. PO got a good deal on it, used it until the motor let go. Wasn't ready or able to buy a new rig so took the path or least resistance getting back on the road. Now it is nickle and dimeing him to death and he...
There is a fella that works at our local Napa that has one on his truck. According to his co-worker he wants to get rid of it!
Can try to get more info if you like...its about 30 min north of Baltimore Md.