Need assistance diagnosing motherboard issues

PC specs are this:
MB: DFI lanparty x58 t3eh6
CPU: intel core i7 920
RAM: 3x2gb corsair ddr3 1600mhz ram
GPU: radeon hd4870
PSU: coolermaster 650 watt RS-650-PCAR-E3

The other day, while playing battlefield 3, my computer just switched off (had been running perfectly for 3+months i've owned it). no blue screen, just off. Long story short, did some digging and worked out the issue was probably the PSU (apparently this particular model PSU is rubbish).

Tested the PSU with a multimeter as per this site's guide, all voltages were within acceptable tolerances. Got to this step:Check Pin 8 (Gray, Power_OK) should be 3~5V to signal the CPU that power is okay to start. If not above 2.5V then signal not high enough to trigger CPU for start.

And sure enough, my multimeter read zero volts. So, i plugged the PSU into another machine i had lying around, and it booted/operated fine- which rules out the PSU as the issue.

When the PSU is plugged in and switched on, the "standby power" LED is lit, but the "DRAM power" LED is not. When the power button on the motherboard is pressed, connected fans will "twitch," but nothing is displayed on the onboard POST display.

What do you guys suggest?

Does all this point to a faulty motherboard?

Was the other machine you had laying around comparable to the DFI? Your multi-meter results still indicate the PSU is a problem, even though it operated the other system. It COULD be the motherboard, but I would try a new PSU before replacing an entire system. Look for 550W-650W at least 80+ Bronze. See proximon's list here at TH.

Yes, that one should be fine. You can get a few more amps with the Corsair 650TX V2.

Alright, so replaced the PSU with the Antec High Current Gamer 620w. Still having exactly the same problem.

Something i've noticed: Motherboard's manual states this regarding LED's:

"DRAM Power LED- This LED will light when the system’s power is on.
Standby Power LED-This LED will light when the system is in the standby mode."

So if I read this right, when the PSU has power, but the computer is off, only the "standby power" LED should be lit, and once the power button is pressed, both the DRAM and Standby Power LED's should be lit?

Currently the POST LED does nothing... I've tried resetting the CMOS, but not since I replaced the power supply.

Will check this when I get home and report back. Cheers