The issue is somewhere on the chassis. I've tried the monitor in a different cab, with a different game and with another remote board and remote board wiring, so I can rule out it being the fault of the game, cab wiring or remote.
The issue manifests sporadically. It's gotten progressively worse though. You can hear a faint chittering (I honestly don't know how better to describe it) on the chassis when it's doing it, but it's impossible to pinpoint by sound alone. I gave all the pots a good tweak and the caps have been recently changes, so it's not either of those.