monitor will not center correctly in some modes

Some resolutions are not centering properly on my monitor. I change the resolution and the auto adjust will run and move the screen way over to the right side, so far over, in fact, that I can't fix it with the positioning settings. I have done a factory reset several times and it has no effect; some resolutions work, most don't. Some are pushed off horizontally, some both vertically and horizontally. I've got Windows XP, have the Dell E171FPb driver installed, have the latest driver installed for my GeForce 7900 GT/GTO and it's just plain screwy, although it wasn't a few months back. Any suggestions?