I remember seeing something similar to this when MSD first became popular.
My recollection is a bit fuzzy as this was forty-plus years ago......
My younger brother had a 1977 Nova with a built-up 350 in it. 11:1, big hydraulic cam, Torker, Holley 750, Hooker headers. Fairly quick for a small block.
He was running the factory HEI distributor with ACDelco cap and rotor, using an MSD 6A box to have more spark energy.
It ran great for a few months and then it intermittently started running bad/rough and finally stopped completely.
When we pulled the cap off, the terminals were covered with green corrosion, the center post was completely cooked/melted and there was a big hole burnt through the rotor.
I recall that we replaced the rotor and cap with the better Accel parts and used a special center post (button) from MSD that had different resistance. We also drilled four 1/2" holes in the side of the cap to let the ionized air out.
I recall that the amount of ionization from having so many sparks going off inside the well-sealed cap was a known potential problem.
That cured the ignition issue for at least a few years.
Good luck figuring out yours!
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