Hey there - seems to be a pretty common issue with the Asus 670 card. Not sure why it's happening, but I am affected too. Have a gander at this thread for some comfort in company: http://forums.nvidia.com/index.php?showtopic=230459&st=0
That's a little disappointing, but thanks for the information!
I suppose I'll have to wait for a driver to potentially fix the issue?