Asus p7p55d-e evo random reboots

Hi guys,

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.

I'd like to sort this out before I RMA my G.skill modules.Any thoughts or help would be much appreciated.
The system is not OC'ed.

CPU :Intel i5 750 with Xigmatek s1283v Dark Knight
MOBO:Asus P7p55d-e evo
GFX: Asus HD5850 1Gb, XFX 8800GT PhysX
MEMORY:2Gb G.Skill DDR3 PC3-16000 2000MHz Ripjaw Series (9-9-9-27)
HD: 160HitachiDeskstar,200Samsung,500WDCaviarBlue,80IntelX25-M SSD
PSU:Coolermaster Silent Pro M 700W
OS:Windows 7 RTM

In Lian-Li PC-A05N