crashing and restarts during gaming.


Aug 28, 2011
Hi guys,
This is my first post and I need some help. I just built a system recently.
Here are the specs:
ASRock P67 extreme4
MSI Gtx 560 ti twin frozr
Intel i5 2500k
G.Skill 8 gb F3-10666CL9D-8GBXL
Antec HCG 750 watt psu
Windows 7 64 bit

I've been having trouble with this system. I can run valve games such as TF2, Portal 2, and CS:S fine. I can run black ops but I get occasional crashing. And if I play battlefield bad company 2, that crashes within ten seconds restarting the system, same with assassins creed 1 and starcraft 2 after a few minutes. I ran memtest 86 on my computer and it froze 3 times. I haven't completed it, so I don't know what to do. Any help would be appreciated.
Thanks in advance