This seems to be situated around LFS in general for me, but my computer locks up and hangs only while in lfs. Of course as you know i had rfactor running for nearly 15-16 hours straight and i didnt get a single lock up whilst playing that. I've played prototype at one point for maybe 4 hours or so, Crysis 2 for maybe 3-4 hours straight. Basically games that are modern and PC intensive.
I've run memtest and came back with no errors, i have Speedfan running and just played crysis 2 for a hour (on max i should say) and upon exiting, my temp was 45 c on the taskbar. I haven't tested with a clean install yet however. Will do that now.