I used to get this with my old CRT monitor. It had a de-gaussing feature, (not sure if I spelt that right), and sometimes it would de-gauss itself automatically and the screen would shake.
Sorry, I see you have a 17" LCD. So scrap that.
I'm out of ideas at the moment. Hopefully others here will know something.
Occasionally I get an annoying vertical text jitter in an on-screen window. This was first noticed in IE6 and now IE7,when present it also occurrs in an MS Word window. It has always seemed to be a hardware failure however, I have changed processor, memory, graphics card with no improvement. Now I have a completely new system as per my profile and still the problem occurs. I can only suspect IE6/7 because this is where I usually encounter it first. A system re-boot is required to fix it and I often get a couple of beeps from the hardware as the system shuts down.
A great puzzle and I wonder if others have this experience.