Why does my iMAC display start jumping left and right by itself?

Update: Since writing this post I realize that it isn't exactly jumping left and right. What happens is that the display is moving a little distance to the left, about and inch but probably not always the same amount, and the portion of the display that goes off the screen wraps around and comes in from the right. In other words the whole desktop wraps horizontally. The menu bar moves with it but the dock does not (I have it along the bottom of the desktop). It must be a hardware problem but is it the display interface or the mouse? (It only happens when I'm using the mouse).

Bingo! I think you're right. I've probably been inadvertently letting a second finger touch the mouse while I'm moving the cursor or clicking with a primary finger. I'm so glad it isn't a hardware problem. I would hate to have to transport this monster somewhere to get it repaired. Now that I know what it is I can probably avoid doing it.

Thank you very much for replying.

I gave up on the magic mouse a long time ago. Partly because of this "stray finger touch" issue and also because mine would occassionally disconnect for a while. No matter what I did (reboot, fiddle with the bluetooth control, turn the mouse off and on, etc.) it wouldnt' come back. And then a few minutes later it would suddently come back and act normal for another few weeks or month. Two Apple guys and the local apple store said "yeah sometimes they do that.. it's a pita". So I switched to a wired mouse that I had on hand and no problems since (except it uses up one of the USB connectors, or which there are only 4).