What kind of monitor? And what resolutions are you
trying to run? I've seen goofy things like that on older
15 inch monitors that simply won't take the settings
above 800 x 16bit ... or if the vertical refresh is 85hz
or higher. Also, on newer monitors, "fast writes ON"
will do that, but it is worse than that. I wouldn't over
think a problem like this. Get a better monitor and video
card.

johns