I always get the same Xid error message:
Sep 30 19:35:14 andy-Inspiron-1520 kernel: [62458.990142] NVRM: Xid (0000:01:00): 13, 0003 00000000 00008297 00000f10 44610000 00000040
Fortunately my system does not crash completely but just hangs until I switch to tty and back to the desktop again. After that everything is normal again except for issues with suspend and resume.
In my case it seems to be related to Gnome Shell as it only happens when the overview mode opens up or closes. I think it started after I upgraded to Ubuntu 12.04. Disabling thermal monitoring with thermal.off=1 or enabling MSI for my GF 8600m GT did not help.