Have you checked for gunk from the air filter box all the way back to the plenum? My old 85' did something similar and turns out it had a TON of crap built up all over the place that I cleaned out and it ran great afterwards, just a thought.
Other than that TPS may be an issue. May want to test the output voltage from the airflow meter (AFM) too.
It should change in a smooth sweep with the movement of the flap door.
You may want to look up the FSM online for your truck and then go through it.
How many miles are on this truck? Lots of simple things like fuel filter, fuel pump etc to check as well. Obviously you want to find the culprit without spending a bunch of money on parts you don't need.