honestly if you want to know what might cause this:
in a cathode ray tube, the devices that produce the rays can begin to short out, or just plain die. The shaking effect can be caused by some that are going bad and going on and off at random.
if your monitor is less than 3 years old, RMA that POS, other wise take it out back and put a bullet in it, its as good as a horse w/a broken leg - the cost to repair that kind of thing (assuming that's the cause) is probably more than a decent NEW monitor, or close enough to make it not worth the cash.
IMHO I would almost bet that's the cause.
When the "shaking" is isolated to a particular area on the screen and the remainder of the image is fine, THATS NOT YOUR VID CARD....it would only be your card if the entire image, or a perfect block (yes, like a square or rectangle) was "making waves" on the screen...or particular little bits were going looney......if you've ever seen artifacts, that's what I'm talking about....w/a vid card its either real bad, real good, or dead in the water.
Go get a new screen. Unless you like the "vibration" effect that wiggle adds for your porn :lol: