hi guys

help required, I have run out of trouble shooting steps.

Symptoms: the pc locks up (freezes) after a seemingly random amount of time.

I originally thought it to be a temperature related issue, as turning the fans to maximum seemed to eliminate it. However I was using NVidia
monitor to log the temperature and it never really went above 40c for the processor, 43 for the system, and 59c for the graphics card.
I replaced the 80mm rear fan with a 120mm one I had lying around but that made no difference.
I replaced the noisy stock processor fan with an Arctic 7 Freezer Pro (because the noise of the fans was annoying me) but this seems to have made things worse! I ran the fans at normal (quiet) speed and although the temperatures reported by the monitor were comparable to how it was before when the fans were at maximum, it soon froze.
I then started running the fans at maximum, this saw the processor temp drop to around 28c, system 40. though the gpu stayed around 58/59. But the computer is freezing even more than before!

The PSU seems quite noisy and I wonder if the new processor cooler is blocking the airflow to it, but it doesn't seem too blocked.
I've reseated most of the components, run memtest (which is fine) and updated various drivers, all to no avail.
Any advice on what to do next?

PC details:

Intel Core 2 Extreme Quad QX6850 3.0 Ghz
4Gb Corsair Memory DDR 800,
500GB SATA Hard Drive,
X-Cruiser Black, Atrix 650Watt psuD,
640Mb Nvidia 8800GTS graphics card DX10,
ASUS P5N-ESLI 1333fsb-DDR800x4 SLI

Are you perchance overdriving your clock?
If you are, I suggest you return the clock to its normal rate.

No, there is no overclocking