From the little info you have provided , i would say its a problem withe the video Card.
But thats just a guess.
Any way , my advice is , remove The ATI Gfx Card, Get 1 Can of Accelerante , Douse the Card Liberally Apply Heat Source (Must be at or above Combustion Reaction temperature of Accelerante ) Do a pagan Fire dance proclaim your loyalty to Mars.
And whey you've done all the Go By a 7 series Nvidia GFX card, On paper Nvidias are not as Good As ATIs , in Bench Marks they are not as Good, But in Life in the Real world they are much More Stable IMHO.