Boy do I sympathize! Had the exact same problems on a couple of my first gassers.

Two different engines and ignition systems (BME & DA) and the same intermittant glitches that drove me nuts. Won't argue PPM vs PCM because that will not solve the problem. Check the simplest things first: On one airplane I had the ignition battery pack too near the alum. wing tube trying to set the CG - moved the pack two inches and success. On the other, the antenna was outside the fuselage, on the bottom, and the loose end would flap against the tailwheel bracket - moved it to the top and all glitches were gone.

These might sound obvious but my attention was diverted by all the advice from "experts" who looked 'em over and said I needed PCM, isolators, chokes, new servos, new receiver, twisted extensions, shielded extensions...yadayadaya.