['76 Big Block]

I got to thinking about this also, and remembered that many moons ago, I had an Oldsmobile that would run like a top, until you tried to accelerate---that problem turned out to be a wire from the pick up coil below the advanve (in the distributor). The wire would stay connected until the advance tried to work, then connection would break or get erratic----let off the gas, and things would be fine.
It's an odd problem, but things happen. To fix that, the distributor had to be pulled to replace the part.
Good Luck!