This problem actually occurred on my dual-boot Win7 64bit too. But the problem on my Windows was not so critical, because Windows will just popup a error message, reloading the driver and everything back to work.
On Linux/Fedora - it cause system not responding, just have to reboot the PC. Once Fedora is my primary work environment, so I want to deal with this problem on Fedora first.
I can’t figure out how the problem come from, I can confirm that the problem only occurred when the web browser (Chrome or Firefox) is running, and I never have this problem when playing 3D games(CS:Source, Killing Floor, Serious Sam: BFE and Unigine benchmark).
OK, I ran memtest86+ today, and the result seems fine.
Also I never got crash when doing some heavier task like video editing/encoding, software compiling and 3D games. So it hard to find out what’s the problem on my hardware.
I have similar problem symptoms with my 560Ti but the freeze only happen when my computer is idle for several hours with or without screensaver. If I keep using the computer I never experience the problem.
I run ubuntu 13.04 but had the problem with 12.04 and 12.10 with various drivers, the latest from 319.23 or the 310 or 304 packed with ubuntu.
I don’t really beleive this is a hardware problem as using the opensource “nouveau” driver my system has ran more than 45 days in a row without problem. But I need the nvidia driver as I use darktable and need the opencl libs.
In my case sometimes hardreboot is the only way but sometimes I can still ssh my machine from another computer and soft reboot.
I found that in the syslog I have always the same paterns of message, see bellow.