As far as the plug cap goes, one thing you can try is using a metal screw-tightening clasp on it, the type used on lots of hoses in automobiles, to keep it from rattling and making LOTS of noise for your receiver. Also, wrap your ignition unit in a couple layers of aluminum foil. And finally, keep all servos, wiring, battery, etc. attached to the receiver at LEAST five inches from anything related to the ignition unit. That includes the throttle servo and its wires, the charge jack, switch, etc. I keep all my radio items in the middle and rear of the fuselage in my gas-powered planes and all of the ignition-related electronics in the very front.

I agree with what was said about PCM vs. PPM/FM. Some people suggest doing your initial range checks with the engine on using an FM receiver so that interference will NOT be disregarded by the receiver, that way you know if you have a problem. THEN, install your PCM receiver and range check it. PCM is great, I use it in almost all my planes, but if you have interference with FM, you have it with PCM, it may just be ignored most of the time.

Just my $.02.

-Pat