I had an "interference" problem several years ago with an older Sachs 3.2. Radio check was fine w/o engine running, but everything would go nuts when engine was running. I did everything I could think of, moved equipment, changed plug and linkages, and rerouted antenna. No luck, so I started removing components from the system. As soon as I took the switch out and plugged the battery straight to the receiver, the problem went away. Worn switch was arcing and losing contact with the vibration.
Have also seen a situation where all was fine and when the model was turned on it's side, there was a major glitch. Turned out to be a broken "filter" crystal in the receiver.