Comment 115 for bug 101943

spiro2903: I think you have been hitted by bug 352218 (marked at duplicate of 95569) due to a problem in frequency scaling reported and confirmed long time ago but still unsolved. A workaround is to set the frequency governor in the "conservative" state. This may slow your computer a bit but it is better than rebooting every now and them.
For changing this, go to a terminal and type gconf-editor
Open apps -> gnome-power-manager -> cpufreq
and set policy_ac and policy_battery items to "conservative" (I think that "performance" also works and is faster for a desktop, probably too power-consuming for a laptop).
Good luck.