My monitor powers off and PC unresponsive - tests done, what next?

Hi all,

My new-built PC has a problem - the monitor powers itself off - generally when the PC is stressed (ie in games - Bioshock and Oblivion both do it, other games are stable) - it won't come back on even if I turn it off and on again, nor will it respond to keypresses/mouse movement. However, the noise of the PC carries on - the lights are still on, it seems it's only the monitor going off / maybe the GPU?

I have done the following tests - I'm trying to find out if it is related to CPU/Memory/GPU etc.
Please can you help me by suggesting what I should do next?

Memory tests:

1 - Memtest86 - Screen went into standby after 15 mins. Would not come back on as above. GPU had been whirring as though 100% (see full specs below).

2 - Memtest86 - Screen went into standby after 40% of the test - about 25 mins I think. GPU had been much quieter this time - I had made NO changes in between, so I don't know why it was suddenly quieter. PC case was cool inside, RAM was cool

Comments - If the memory had failed wouldn't it have come up with a different type of failure rather than the monitor going off? I mean, Memtest really does nothing to test the GPU, so I'm confused at this result...

CPU tests:

1 - Orthos (small FFTs - stress CPU) - Ran for only 1 minute! Then the same thing - monitor powered itself off, would not come back. However, this test was done very soon after the ones above, so I repeated it -
2 - Orthos (small FFTs - stress CPU) - Having given the computer 30 mins to cool down. Stable after 54 minutes - in fact it's running now.

Comments - think this rules out a CPU problem - this is good!

GPU test:

1 - Ati Tool was stable after 1 hr of artifact testing

Comments - this rules out a GPU problem, right? :-S

PSU tests - is there a way to specifically test the PSU? I know very little about PSUs to be honest and could use some help.

Motherboard tests - are there any tests specifically for the mobo?

Specs:
CPU: Dual core 2180
GPU: 8800 GT - Point of View
Motherboard: Gigabyte P35 DSL
Hard disk: 1/2 TB Samsung HD501LJ
Memory: 2x1GB Corsair XM2
All new
Power Supply: Not a good one, will check details when current test finishes!
All are run at STOCK thoughout these tests. No overclocking or anything like that, although I did O/C the CPU for a bit a couple of weeks ago.


Thoughts most welcome.

Gear

OK my PSU is a 350W Sumvision piece of rubbish - I am gonna upgrade it. I had thought it was a 450 but it seems from http://extreme.outervision.com/PSUEngine that I need 323W + another 30% for overhead. It could easily be this holding the system back and I need a spare PSU anyhow to make the PC for my sister out of the old stuff!

OK - off to ebuyer for now.