OC - Overclock.
Could be a number of things, you need to do some testing of memory, cpu.
1st find out the temp of your cpu by looking in the bios and download Realtemp, cpu could be overheating.http://www.techpowerup.com/realtemp/
(Scroll down page for download tab.)
Also download occt - http://www.ocbase.com/perestroika_en/index.php?Download
What you want to do is open Realtemp and run occt for 30 mins and let me know how hot it gets and if it crashes.
Could take a bit of working out but I'll try and help, if your not sure of anything let me know.