checkerboard fuzzy screen then freezes
whenever i start up my computer after about 10 minutes (sometimes quicker) i get a fuzzy screen where there are fuzzy rectangles and clear rectangles in a checker-board type style. initially it's possible to click on the desk top, then back on the open window and still use it, but then the clear parts go fuzzy too, and the circle of death comes up. Won't go to sleep, just have to power it down with the button on the back. any idea what's wrong?