external monitor image jiggles
The image on my external monitor jiggles. It jiggles a lot. Almost constantly.
MacBook Air, Mac OS X (10.6.7)
It might be just a bad cable. The monitor itself could be going south. It could be the system board of your Mac, or the graphics hardware on it.
Take the monitor to any other computer. Does it do the same thing there? If so, did you use the same cable? If that is also true, use a different cable and check again. If it still jiggles on another computer with a replacement cable, then the monitor would be the main suspect.