That's pretty common. The first place I'd look is plug gap. Contrary to all of the marketing hype, you really don't need more than .040. I used to be a .055+ guy but I've changed my evil ways.
You could also look into the plug wires. Anything that makes it more difficult for the spark to jump the plug gap can make it do what it's done under your cap. Should be an easy fix
Edit: I probably shouldn't but where's Jeephammer now to defend his beloved Duraspark and all of his HEI "red dust" comments? Ok, my bad. It's just a product of the spark not going where it should. Good wires and the right gap will probably fix you right up. No worries.
Edit 2: You've changed the metal composition with the spark blowing particles out of it. I'd replace the cap and rotor (upgraded, brass posts) and try to nail down what caused it to begin with.