I have the same problem...with me, it's the huge power lines outside my window.
Do this test: Set your refresh rate to 60Hz...see if the problem persists. If it stops, there's likely some sort of electrical interferance (since power runs at 60Hz...this solves it for me). Also, try (if you have the money) getting a UPS with automatic voltage regulation. Sometimes monitors will jiggle if you have unclean power going to the system.
If 60Hz doesn't help, and a UPS doesn't help, and you have no obvious electrical sources, it's likely that the tube is going in your monitor, or that you have some sort of magnetic interference (speakers, etc).