Just out of interest, are you running the cards faster than their rated capacity? This would exacerbate any heat problems you might be having, making your system unstable and prone to random crashes (especially if you have increased the clock speed and haven't taken precautions against heat).
Even if not, from what you're saying, I'd take a look at some extra cooling options.
And take a look here for some interesting info:
http://www.driverheaven.net/windows-...-properly.html