We used to call them "Phantom Gripes". Worked great on the deck but computer dumps in the air or system off line. Also check the Y-harness. Do you have a engraver or something that vibrates so you can touch the gear/harness/receiver with?
Many years ago in high school, I used to work at a Kraft Radio repair depot and that is how we would find many receiver problems. They would work OK on the bench but fail w/ just a touch of vibration.