I had a problem very similar to that on a 77 Chevy Blazer that I owned. I built the truck from the ground up. I use the HEI distributor, new rotor and cap. It left me sitting on the side of the road many times.
Turned out that the rotor, even though new, was losing contact between the center contact on top and the brass tip. I refused to believe it was the rotor because it was new. New doesn't mean anything. I finally found the problem with it when I took a used one from a distributor and replaced it. It looked perfect right up to the time I threw it in a trash can.