1993 Chevy Truck Engine backfires and has no power at drive

My truck has a Z-code 4.3L V-6, no emission air pump, A/C, TBI, all stock. It starts and idles fine but when I'm on the road at 55-60 mph and try to accelerate it has a very noticeable hesitation and occasionally backfires. Took it to the local Chevy dealer and they said it was the EGR valve so they replaced it. It still does the same thing. Did the normal routine. New plugs, wires, checked distributor and rotor (both look fine), new air filter, cleaned the injector fuel screens, new fuel filter, verified correct timing, nothing seems to help. No codes on the ECM so I'm totally lost. I have an Actron code scanner and I did the service diagnostic test with the engine running and it indicated an "open loop" until the engine warmed up, then it showed a "closed loop" mode which is supposedly normal when the engine reaches enough temperature for the O2 sensor to feed back a signal to the ECM. I checked the input voltage to the MAP sensor and it was within range. I am at a total loss as to what to try or even check next. Any help would be greatly appreciated.