I have had several ignitions fail and always the failure was intermittent. One failure was that on hot days after more than 10 minutes into the flight, the spark would quit on one cylinder (on a twin engine). Let it cool off for 20 minutes and then it was fine again. Another unit failed completely, but only on one side. I could swap the two ignition wires and see that the engine was running on the other cylinder (one cylinder cold and one hot).

One failure was such that the ignition was putting out so much RF noise that the throttle servo went crazy. The engine was running OK (spark was OK) but the throttle was surging back and forth. I thought it was a bad servo so replaced it. Same thing. A third servo yielded the same result. Replacing the ignition solved the problem.