My system has a PNY NVidia GeForce GTX560 Ti, which is brand new (the whole system is) and it has this problem, too.
As far as I can tell, the card is not overheating. It shouldn't be, as it's not been doing much whenever I've seen the problem. I certainly haven't seen it happen while running applications with a heavy graphics load (games).
The problem has only occurred three or four times (that I've seen) in the three weeks that I've been using the system.
I've got the latest drivers from NVidia, and I've not seen the problem occur in Ubuntu Linux, which I use a lot.
Thanks,
John