Does it happen right away? If so, try running pc w/minimal hw/sw, ie: only 1 stick of ram, vid, kb and mouse,, see what ya get.
Add stuff as you go along watching to see what causes problem.
Before you start, reseat everything.
If yo got other hw, try using them when you get to the problem, (you already tried diff vid), ie: monitor,kb etc...
Pain in the behind I know, but should pinpoint cause.