I had that problem once. It happend to me when a 10 cent coin was on my graphics card. See if there is anything on your card. Dont ask me how that coin came to be there.
Ok, well it ran all night (about 14 hours) with no prob, so I have taken the Graphics Card out and left it to run with the onboard card. I will see what the result is this arvo and post here.
I came home, ran to my room and discovered my computer, crashed. I'm going to take the RAM out and test it and if that dose not work I can't really think of anything other than the MoBo.
This is a test of the Emergency Broadcast System..... In my opinion, I think it's the video card because I got a similar screen when I over-overclocked my old Gigabyte ATi 9600 Pro. Ya, try underclocking it or see if it has to do with temps/voltages or a combination of both.
Edit: Wow..... thread was longer than I thought it was. Well I would still try temps and voltages....
maybe a certain spot in your mobo is totally fried, or try setting a larrge fan next to your computer with the side off to see if it makes a differance. if it does, logic tells me that you are overheating it somehow