my monitor does this MILDLY.
Here is why:
It as against a wall. On the other side of the wall is our main television, a standard CRT type.
When the TV is on, the electromagnetic feilds that aim the electron beam at individual pixels interfere, so my picture gets shaky. It is less noticable on the TV.
ANY electronic device can do this. ANY at all. for example, I was working on my sister's comp, and I set her alarm clock on top of the CRT to get som more desk space . . and skakiness insued.
so, speakers, other CRT's, anything that is plugged in . . they can all cause it.