Sounds like sediment somewhere floating around and blocking when it wants to.
Ballast Resistor, as used I our older trucks, is in the "run" circuit to lower load on points.
When starting, it is bypassed momentarily. There is a straight voltage to coil while big draw starter motor tries to take all of it and lessens the spark needed to start truck quickly. But a straight voltage/current to coil/points during driving (run) would accelerate point burn out. So they say. I've read where it is used to balance out over voltage but I thought voltage regulators did that.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn