sounds similar to a problem I had with an HEI distributor.

My problem: the little tit on the pick-up coil which is supposed to orient the coil, had broken off ( somehow ). This caused the coil to move slightly back and forth. Eventually, the wires cracked... and would ground out intermittently.

I bet you have something similar. In fact, I bet if you dumped that ancient technology dual-point distributor in favor of a brand new HEI, your problems would dissapear.