John, that #7 lead on the cap might be a sign. Why don't you try this: Get a new cap--save the replacment wire bit for later. When you install it, load the boots up with dielectric grease; also, use some d/grease between the coil and rubber cusion that holds the center connector on the cap. (Unlike the boots to cap connection, be a little more sparing with d/grease.

The hei throws a lot of fire, and even with point distributors, the fire can be squirrely. You might have a condition whereby the distribtor shorts out. It's a combination of s/happens and Muphy's law.