I have a 1994 Chevy Cheyenne 6 cyl. Standard. When I accelerate the engine seems to bog down but when I let off the gas it seems to kick back in. When I push back on the accelerator it bogs down again. I have changed the fuel filter, air filter, spark plugs, plug wires, and distributer cap.

. Replace the Ignition Control Module:

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. Only use the exact ACDelco plugs (likely #41803) that came in the truck. Stay away from cross-reference plugs.

I'm going to guess that this symptom is more prevalant once the catalytic converter has gotten fully heated.
If so, the converter has gotten congested and needs replaced.
What causes this? Many things can contribute, but often poor gasoline is the culprit.

If you don't know your gas, read this: Top Tier Gasoline
... and you thought the price difference was cause they were greedy.