Help diagnosing mobo/PSU issue please

[MSIC]

Am desperately trying to solve a problem with my friends computer. I built it for him as a favour, he paid, but it's had no end of troubles and had a few RMA's of components.
Having recently recieved a new PSU (which i felt was previously faulty and the supplier agreed) I now have similar problems, and am tearing my hair out.

Components
Aspire x-Qpack, with the standard PSU as follows:
DC Output +3.3V: 31A , +5V: 29A , +12V1: 14A , +12V2: 16A , -5V: 0.3A , -12V: 0.8A , +5VSB: 2.0A
+3.3V, +5V combined load: 200W
+3.3V, + 5V & +12V Combined load 430W. 24Pin connector.
Motherboard = Gigabyte GA-K8N51GMF-9 (micro-ATX Athlon 64 939). 24pin power connector.
CPU = Athlon 64 3200+
RAM = 2 x 512MB Corsair Value Select
Hard Drive = Samsung Spinpoint 250GB PATA
DVD ROM = NEC 3500A
Also front multi-card reader.

The Problem
The machine will sometimes turn on & work, & sometimes will not power on. Sometimes waiting for some time helps, sometimes no obvious wait helps. It is not repeatable on demand and i havent yet identified a definite pattern, despite the machine being like this for a couple of months. I have tonight noticed however that unplugging the extra '12V' power plug does enable the motherboard to recieve power, as the CPU fan will spin up when the power is turned on in this state. If you turn it off, plug the 12V lead in, and turn it on again it still seems to work, but will at some later point not work again...

I have ruled out
Most likely not a fault with the hardware as such. I have RMA's the motherboard, and seperately RMA'd the PSU. With three different combinations of seemingly working components, the same issue occurs.

My opinion
Well i dont rightly know, otherwise i wouldnt post here. But it seems to be a power issue. In a previous post someone suggested the motherbaord could be shorting against the case. I have now used little cardboard washers on top of the motherboard, between the motherboard and the screws to hopefully rule that out. Its almost like there is some sort of power build up, and it's not being grounded properly....but i'm no electrical engineer so i dont really know.

My Request
If anyone can help me out on this one, i would love it, truely. I can see this not getting sorted, my mate asking for his £400 back, and me being out of pocket after trying to do him a favour!

[ash_rm]

When it does turn on, is it 100% stable? Has an operating system been installed?

You could try running the system outside the case to eliminate any doubt of it shorting out.

[BUFF]

have you tried it with a quality PSU instead of the standard Aspire fitted one?

[MSIC]

When it does turn on, is it 100% stable? Has an operating system been installed?
You could try running the system outside the case to eliminate any doubt of it shorting out.

When it turns on then yes, it remains stable. Win XP works perfectly, no crashes or lock-ups. The only time it doesnt work is (sometimes) when you have to turn it on.

have you tried it with a quality PSU instead of the standard Aspire fitted one?

Tried just powering it up with my main machine's PSU,a Tagan. Worked perfectly first time, although then again so did the Aspire as soon as I switched back. I certainly couldnt use the Tagan full time because a) its mine b) it doesnt fit the case.
If the PSU is at fault, it's design issue as this is the second.

Edit: A friend of mine who read this has suggested that I simply try running the whole thing without the 12V power connector plugged in.
Is this feasible? I thought it was needed for stability? Anyone else done so?

[magicrichard]

I had a really similar problem, in the end I replaced the power supply for an Enermax one, and havn't had any problems since...

[MSIC]

Did the enermax fit in the Aspire X-Qpack case ok? If so, which enermax?