88 4x4 engine problems?

So my 88 K1500 runs just fine out the gate but after driving about 20 minutes it develops a miss in the engine. Techs thought maybe the O2 sensor but the comp test didn't denote the O2 code. I'll be changing out the 02 anyway and hope that remedies the problem but was wondering if anyone else might have the same problem. Check Engine light comes on after about 10 miles of steady driving. Any thoughts?

If you checked for codes which one(s) came up??

do a tune up(plugs, wires cap/rotor, fuel filter) try unhooking the vacuum line at the egr and plugging it if it still misses.

The shop tech didn't mention any codes but did tune it up, new rotor, dist cap, plugs, wires, etc. A whopping $300 later and still has the problem. I'll check the EGR as suggested. Thanks!

The shop tech didn't mention any codes but did tune it up, new rotor, dist cap, plugs, wires, etc. A whopping $300 later and still has the problem. I'll check the EGR as suggested. Thanks!
cap+rotor: $15, plugs: $10, generic wires: $25, 1 hr labor. someone raped you on this 'tuneup' that didnt even involve a cursory scan tool hit.....thats the first thing I do when a car comes in and someone says the light was on. if the light was on, you have at least 1 code...