The electronic ballast resistors are cheap, buy a new one and put it on. If it doesn't make any difference (I'd be surprised if it does) you have a known good spare to put in the glove box. I carried one in the glove box for years (never used it though).

How old are the plugs, wires, cap & rotor? All will show signs of failure first when hot, or wet. I'd look inside of the cap first, then look at the wires where they contact stuff for hot spots. Gene