ASUS A6Vc Screen Problem

I have a laptop Asus A6Vc model. When I using any of tasks (game, internet etc.), the screen suddenly turns comletely WHITE! But the task is continue, not deadlock. The problem time is not defined, sometimes after the long time, sometimes in a short time. I checked in dxdiag, but when I testing on Directdraw or Direct3D, it gives same bad result, I mean LCD turns completely WHITE screen. I've found temporary solution that is pressing "FN+F8" (it means change the screen mode LCD to TV, and it normally doesn't find the tv if there is no connection with tv, and it turns back the LCD, this is normal), after this point the screen turns back to normal posetion, but it is not a true solution. My graphic card is onboard and Nvidia Geforce GO 6200 TurboCache. So; is it LCD problem or Graphic card problem? If you know the solution, please help me...! Thanks.