I still have both cap and rotor, so when I get a chance might take a good look at them both.. The cap, from what I saw, looked pretty good and I do remember thinking the rotor looked like it might have some corrosion on the contact point.. I did lightly sand it, but it didn't work..
this same issue happened on my first TR7 back in the early 80's.. Simply changing rotor and dizzy cap fixed that one, as well..
Friendship is like peeing your pants.. Everybody can see it, but only you can feel the warmth..