Irritating hang after upgrades - advice pls

[Iniar]

Evening all,

Couple of years ago I bought an ocuk overclocked system: C2D6600, 2G Geil Ram, ASUSAW9D-MAX and a BFG 8800GTX.

I install Windows 7 Home 64 bit, all ok
The gfx card went so I replaced that with a Zotac 470GTX (future proofing see) and all OK
I then source anohter 2G of Geil ram identical to the pair of sticks I have, all ok
The issue comes when I drop in the Q6600 CPU.

Windows auto detected and loaded drivers, temps running OK in the high 30's, but after 1 or 2 mins, everything hangs.

What should I be looking at to resolve this issue?

I havn't played with the BIOS at all so it still has the ocuk O/C on it. To my untutored eye the BIOS settings are:
266 external clock, x9 multiplier, ram settings 'by spd'

Can you point me in the right direction to try and identify the issue please?

 

[RJC]

As you added extra ram you might need to adjust your overclock.

Also increase the MCH Voltage (north bridge) in the BIOS notch or 2

 

[Iniar]

Found the setting to run everything at 'factory default'.

Without wishing to appear unworthy of the forums, would this be a good place to start do you reckon? i.e. nothing overclocked at all.