Video gets lines, then shuts off and locks up

Lately my computer seems to freak out when I play any 3d game, or watch some movies.  What'll happen is everything will seem normal, then horizontal lines go across the entire screen, and if i don't close whatever app thats causing it in 2 seconds, it shuts the screen off, and locks up.

I've done all I know to try and fix it.  Removing/reinstalling drivers, using different ones, turning down settings, anything i can think of, but it keeps happening.  Does anyone have any idea whats going on?  I'm running XP sp2, with a TNT2 64.

I have noticed that heat seems to be a factor, but since i put an AC in the room, it's been mostly fine.  But it just randomly tweaked out tonight.

Also, if you are in an even latitude remove the case from the right side, otherwise remove the case from the left. Most short-circuits are caused by not following this very simple principle.