1993 GMC Truck low end misfire when accelerating

My truck is a 1993 z-71 with good oil pressure and temp. It will act as if a plug wire is off when accelerating from a low end with low rpms. I can make the truck accelerate without missing but must very gradually accelerate. Pulling a trailer is out and so is pulling out in traffic. In park, you can accelerate without a miss by keeping the oil pressure over 25lbs. At idle, on warm engine, I have 15-20lbs and at 2200rpms=40-45lbs. I dont know if this is the heart of the matter, it seems more of a firing or injector problem to me. I took it to a local mechanic and he reset the timing and it helped but he said he couldnt fix or diagnose the problem until it became a problem or a warning light showed on the computer while he had it in his possession. Please help, I need my truck back!