1988 K2500 Chevy 4x4 5.7 misses under a load while driving but not while revving the engine?

Chances are you have a bad wire or plug causing it ; It is easy for a weak plug or wire to hit without a load ,so if it is time for a tune-up I would replace the wire set and plugs. Check your cap and rotor also. I hope this helps you. Mark Mark thanks for the help. New wires did the trick. Truck runs great. Thanks again!!!!