I'd suspect points or condensor....maybe the distributor itself. I don't like points, so if it were mine, I'd put a Pertronix kit in it.
I had the same type of problem with my 64 292. I finally replaced the distributor with an HEI. No more problems. In my case the distributor bushings were worn (which contributoed to the problem), so it was easier for me to install the HEI than rebuild the distributor and install the Pertronix.
