What is the exact make & model of your PSU?
 
Also, try the latest beta driver 320.00  I find it to be a much better, more stable driver than the one you're using.  Make sure to do a custom install only installing the graphics & PhysX portion of the driver.  Don't install the audio or the 3d aspects of the driver.  Put a checkmark in the "clean install" box as well.

To add to bdary you might also want to wipe all of your old video drivers and do a fresh install. 

The beta drivers seemed to do the trick. I was beginning to think it was the card when I tried several previous versions but all seems to be well now, thanks guys!

FYI --
I was having the same issue for months with my GTX570 -- it never crashed my system, but it was annoying as heck!!  I tried everything, removing drivers, clean driver installs, etc.  Nothing, and I mean NOTHING, corrected the problem.  So, I purchased a GTX670 FTW and installed it in my system.  Guess what?  EXACT same thing.  About every 5 minutes, even when doing nothing at all, my screen would go black for a couple of seconds and then come back, with a popup by the system tray saying "blah blah blah has stopped responding and has been recovered".  What are the chances of two different video cards being bad?  No, had to be something on my system.
 
Well, I ran Windows Update manually on the 15th of April, and I have not had ONE SINGLE instance of this behavior.  That's right, 3 weeks later, system hasn't been shut down since then, and it has not occurred once since I did this.  Unfortunate thing is I have no idea which of the updates fixed things, but it was obviously something in the update process that corrected the issue.