I had a problem like that before on my old pc, and it turned out that it was my ATI all-in-wonder 128 video card. Once your video card starts to get old it tends to do funky things. What kind of video card do you have? When was the last time you updated your drivers? Have you noticed any other strange behaviour? Also, you should hook your monitor up to a friend's computer and see if perhaps it's the monitor itself going shotty. There's my two cents
-Matt Rock
"Hell is an Irish Pub where it's St. Patrick's Day all of the time." ~ Christopher, *The Soprano's*