At this point in time I have no idea it might be a memory issue your graphics card could be going bad you said you have a nvidia 6800 does it have any memory included have you ever overclocked it. You really haven't provided much information to go on so it is all just a guess. There is a reason we ask for information without the proper information it is all just a guess. Sometimes even with the proper Info it can't be figured out.

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A little common sense goes a long way

Later Grandpa