I've seen a v8 distributor where the internal components were not clocked properly. This resulted in misfire at certain combination of rpm and vacuum. I think the effect was the rotor wasn't lined up with the distributor cap and the spark was lost trying to jump the gap.
I don't know the root cause. Twisted shaft, sloppy weights, too much or little vacuum advance, not sure. I swapped distributors and the problem went away.
If you are running points you might try adjusting them either open or closed a bit from spec and see if it helps. Reset timing of course. This should change the relative position of the rotor and cap. Or you could alter or remove the key from the bottom of the cap so you can rotate it on the distributor body.
