84 C20 - Intermittent Rough Run/Black Smoke

Got an 84 C20 with a 350. Truck is back and forth between running great, and sputtering with some smoke coming out.

Truck is hard to keep running it sputters so bad. Tune up done already. Running a bit rich, but that doesn't usually cause this problem.

Truck sat for four months, ran it almost out gas then got more gas, all during a 110 mile trip. Even the stop and go traffic didn't phase this thing. Parked it for a few hours, went to store, when it happened down the street I figured I didn't let it warm up enough, but now happens warm, hot, or cold.

There is a black residue under the tailpipes only when it smokes black, and it only smokes black when sputtering, Clears up after between 5-20 minutes of me keeping it running, then runs great until it sputters again. Can go hours of constant driving before sputtering again.

Seems to be some correlation to when I nail the throttle, driving or parked. Also seems to clear the sputtering issue by removing the air filter cover, running rich though, I'd expect that to help it run smoother, but this doesn't make sense.

Could it be bad gas? Dirty gas?

Got to be one of those and I figure dirty since it comes and goes. Need to clean it quick, but can't remove the tank to do it.

Should I just run the gas out of it until it dies, then refill with seafoam or stp mixed in? Worried if it's dirt, that the dirt will screw up the motor being that much.

Any help appreciated.