[Steeveedee]

Thinking more about this, what do the wire running to the points and the breaker plate to ground wire look like? Check the ends especially to see if there are a bunch of broken filaments. It could be that there are enough for idle and a bit of speed, but not enough to carry current when the engine is under load. I don't think that this was mentioned before. The breaker plate ground wire flexes as the vacuum advance moves, so it can eventually wear out.