Question : Problem: Custom Built PC Freezes. Troubleshooting Assistance Please!

Hi

I put together a PC 6 months which has always suffered from a random and occasional freezing problem, but now that problem seems to happen ever more regularly and its costing me a fortune in swapping out parts and i still havent got to the bottom of it.

Base Hardware:

Quad Core Q9450 2.66ghz CPU
4Gb OCZ Reaper PC2 9500
BFG nVidia 8800 GPU (pcie 16x)
Asus P5N-T Deluxe Motherboard
Tagan 1100w PSU
Vista Ultimate

As you can see, it's pretty high rated h/w.
The problem is that the PC will just randomly lock up, with everything freezing.  It remains on, but completely locked up.  All you can do is hard reset it.  If I turn off the screen and turn it back on again, you get a no signal message.

So far I have replaced the graphics card temporarily with a lesser pcie card, problem remained.
I then replaced the original Q6600 2.4ghz cpu for a Q9450 2.66ghz and the problem still remained.
So, I replaced the original 700w PSu for the 1100w PSU and the problem stopped for 48 hours, I thought I'd cracked it, but this morning it locked up again.

I've run memtest on the RAM and its all fine.  I've never had a blue screen or have the pc reset.  It runs at a steady temperature around 40 degrees.

There's only the motherboard to replace now, which i'm happy(ish) to do, but i've spent a fortune on replacing this kit , I cant return the original stuff as none of it appears faulty, so I'm reluctant to spend another £200 on a motherboard and find out that its software related.

Can you help me troubleshoot this issue please?

Thanks

Answer : Problem: Custom Built PC Freezes. Troubleshooting Assistance Please!

seems most likely to me...