Chevrolet Repair: miss in 91 chevy pickup, distributer cap, air control valve

Question
I have a 91 chevy 1/2 ton pickup 350 engine.
the service engine light started coming on and it started missing really bad. so we changed spark plugs helped little but still missed. we changed wires and distributer cap and rotor cap. Still didn't help changed o2 sensor still no change so we changed air control valve and tp sensor and pcv valve.  The light still comes on and it runs rough but when it goes off it runs smooth. but it want stay off. and it idles real high. could you help

Answer
Hello Mary,
I appologize for not answering the question sooner.
I was out of town, and signed out on vacation, but the All Experts system let me down, and didn't get my vacation posted.
There are a few things you missed, but all of that stuff can be checked before buying a new one.
Also, there is a system to read stored fault codes, which can point you in a certain direction.
The EGR valve, if seating on carbon can cause it to act like that, and it can and should be cleaned.
A vacuum leake can also do it, so check all the hoses, and the brake booster.
Van