PC Random Shut Down (New-ish set up)

ASUS M4A87TD/USB3 Mobo
AMD Phenom II X4 840 3.20 GHz CPU
4 gig of Komputerbay (Yea I've never heard of them either) RAM 8-8-8-24
2 x Seagate 500gb hardrives Radoean HD5570 GPU

My PC randomly shuts off (and restarts) with a sharp audio buzz sound. This accurs whilst playing games using other programs or been idle.

This is a fairy new set up (All new parts) and has had windows 7 32 and more recently 64bit OS. (I want to add more memory) This problem has happened on both version so I'm fairy certain the OS isn't the problem.

I've run tests for over heating on the Mobo, GPU, HDD and CPU and none have peaked 40'c (Usually alot lower).

I've firstly orded some new RAM which I defiantly know my motherboard supports, but until that arrives (won't be for awhile) I won't know if that's the problem.

I was wandering if anyone has any ideas or similar experiences with something like this happening? Or what steps should be taken for a process of elimination?

Many thanks Rye.