Radeon Asus 6850 Furmark Crash

Hi ! I buy my new graphic card Asus 6850 HD , and i have a big problem . After 10 minutes o play or Furmark Stability test i have big screen crash . Somone know how to repair this ? i using standard clock of memory and gpu 790/1000 Check it out . My PC configuration : AMD Phenom II x4 965 BE 2x2gb Kingston hyperX blue 1600 Corsair 750W MSI-870A-G54