All kinds of ideas listed so far, and all good ones to!. So to add some "out of the normal" problems I have seen on coil type ing. issues. One thing to try, is watch the coil, and dist in the dark, you maybe surprised to see sparks at places you never thought. I have found both coil and dist cover problems this way. The smallest carbon path in the wrong place can cause all kinds of problems. I posted it before but now I found a picture to add, take a good look at the copper strap on the points, I am here to tell you I spent hours working on a dist "shorting out" problem, no matter what I did it would start to work and then fail, it was all caused by the point strap that would touch the case whenever it felt like for just as long as it felt like. It about drove me crazy (maybe a short trip to crazy anyway!).
Model a points.jpg