dual point distributor

I had to re-wire the top of mine about 2 years ago. The points were grounding out completely.

I agree with cudamark, it could also be dwell on one side is set too narow and when RPM's go up enough it causes it to act as grounded.

Check the wire around the breaker plate for cracks and grounds. Use a meter to check to make sure you have continuity and non-continuity. when points opened or closed.

If wiring checks out, Try getting it to run/vac/rev with just one set of points first. I usually take a paper match stick, or a piece of a match-book and place between the set of points I'm not working to. Adjust one-side and then the other. I like to get them to exactly 30 on each side. Then when combined, total dwell should bump up to about 38.