Homebuild turning off randomly

okay, so here's the specs:

PSU - Arctiv psu, 700 Watt
CPU - AMD phenom II, Quad Core black edition at 3.4ghz
MOBO - Gigabyte SKT-AM3 M68MT-S2
Ram - 2x4 ghz of corsair ddr3 ram, 1600mhz.
HDD - (Forgot the make, it's just a 500GB one )
GPU - HD 5770 ..

Okay guys, so the problem is, that my newly built pc (Built yesterday) just randomly turned itself off. since it's running a stock heatsink on the cpu, im constantly monitoring the temperatures, and the cpu hasnt went over 40c
The gpu is usually around 45c
So, i've ruled out overheating as the pc turned off just after i checked the temps.

The motherboard, Cpu and ram are all new, and before installing them yesterday, i've never had the pc randomly turn itself off.

I've also tried to drain the capacitors on the mobo(I think that's what this does) by unplugging the psu, keyboard, mouse,monitor ,Ethernet cable and holding the powerbutton in for a few mins.

the computer is running a fresh install of Windows 8, all drivers that came with the new parts are installed and (after trouble) the gpu drivers are installed and functioning well.

The pc has turned off 3 times today.

When it turns off, there is no blue screen, no black screen, it simply just cuts power to the pc and all fans stop spinning, etc. I simply just turn it back on.
I'm worried incase this causes any damage to the cpu.