ORIGINAL: preivers
I too feel vibration influences on the receiver are the cause of all evil in this case, plus the untwisted wiring, which passes all glitches unrestrained ovet the entire radio system. Twisted wires function like a choke, and prevent a lot of RF system noise.
I disagree. Twisted wires do not function as a choke. Twisted wires cancell transient cross coupling in wires. They are no help in RF interference problems.
The most likely cause of trouble in gas models is leakage of the interference from the spark plug wire. Make sure the connections are good with no breaks in the shielding and the ignition system is grounded to the engine.
Vibration can cause trouble in any component even if it is new. As suggested above remove components one at a time and check. Pay particular attention to the switches. Just bypass them one at a time and check.