Diagnosing cylinder misfire

Okay, we swapped out the plugs. Fired right up. Running strong again, then shut down on its own. Would crank but not start up. Swapped out the ballast resistor that was already new just a few days before. Anyway after swapping it out, fired right up and runs strong. Set the initial timing on it and idles great, nice throttle response. Something is knocking the ballast resistor out randomly and frequently. Haven’t even got it out of the shop yet and it’s gone through three resistors. The brand is “standard” or stp. May have to try a better brand, it didn’t want to start on an msd resistor. At least getting it to run strong.