Just what the hell is going on here. It does this for up to 5 minutes at a time, seemingly at random. There's no disruption to sound or anything else (that's my shower running in the background).

Sometimes, if I hit the top of the monitor or squeeze the sides, it stops doing this. Sometimes it makes it a lot worse. Sometimes it just stops by itself. I've checked the cable at both ends and it's fine.

It's a Princeton monitor. I've had it for about 2 and a half years and this crap started about 4 months ago. (less info)