I have a 1990 K1500 with a 5.7 liter engine and automatic trans. At speeds above 35 mph it misses significantly while accelerating with medium or light pressure on the throttle. If I floor it it vanishes. I have changed out the spark plugs, cap, rotor, and both electronic components in the distributor. The timing is right on. The miss seems to be much less pronounced when the engine is cold and gets worse as it warms up. what's going on?

 

What I would try next is in the evening when it's dark, have someone accelerate your engine at about the PRM that the missfiring problem is the worst and look for the plug wires shorting out or arking. You'll have to undo any under hood light bulbs before the test. Be very carefull as well checking anything by hand during this test (fan, spinning belts, good jolt from a bad plug wire, accidents happen) best to have a flashlight handy. Good luck.