I recently built a new system,and I am getting random reboots in 3dgames. It doesnt happen everytime, or within a specific time limit,
or specific games. I ran OCCT to stress PSU and GFX and temps seem fine and no reboots. I had in 2 x 4GB G.Skill DDR3 PC3-16000 2000MHz Ripjaw Series (9-9-9-27) Dual Channel kit for Intel P55 modules and system would not post as MemOK! couldnt get stable timings in two of the DDR slots,the other two booted without hitch. The one module kept giving problems with windows not detecting and sometimes not booting, so I removed it.
I ran Mem86+ on the remaining module for 4 passes no error.
Did you enter the correct voltage and latencies in BIOS, and use the dimm sockets according to your manual?