1992 S-10 Problem

Hey folks,
My brother has a problem with his 1992 S-10 Blazer. 4.3, 4X4, 5-Speed
If he just jumps in it, and drives down the road, it will either stall or
run like crap. He can shut it down and restart, and all will be fine.
If he starts it and warms it up a couple minutes, even in warm weather, it will give no problems.
He recently changed the fuel filter, and a new distributor last year, the engine has over 100,000 miles, and runs like a top most of the time. He also recently told me he had a code 42 as well.
My thinking it is something in open loop, but what? I can't find a vacuum leak either.
Thanks for any help in advance,
RJ in WV
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