RE: CRRC 40cc kit
The Champion resistor plug is RCJ7J. The plug that comes with the engine is not resistor.
I have no interference problems because I am using 2.4Ghz. Are you using PCM. FM does not work well with gas engines.
Also you can try securing the plug cap to the plug with a hose clamp. On occassion the spring on the plug cap will not be tight enough and let the cap vibrate against the plug.
Also make sure that your receiver, receiver battery, servo wires, switch wires, etc. are at least 10 to 12 inches away from the engine and ignition module or ignition battery.
You should also be using a plastic or non conductive pushrod for the throttle servo.