Ideas on Power/CPU/MOBO Issue?

Any help or ideas would be greatly appreciated on this one as I really don't want to order the wrong part. So here goes...

Specs:
Asus P5N-E SLI MOBO
Intel Core 2 Duo E4400 CPU
OCZ 600w StealthStream PSU
4GB GSkill DDR2 (4 x 1GB)
1 x XFX GeForce 8600 GT 512 MB DDR3
LiteOn DVD+R SATA
2 x WD Caviar 160GB HDD

I have the memory timings tightened from 5-5-5-15 to 4-4-4-12, and I have my FSB slightly OC'd unlinked from 800mhz stock to 1000mhz. All tested stable under load and cputemp never reported a temp above 43C.
My System is aircooled with a Zalman 9700 on the CPU, 2 x 80mm intake fans, an 80mm exhaust on top, and a big 120mm exhaust in the rear. I built it about 11 months ago and the only problem I ever had with it was about 6 months ago. The MOBO died on me, the same brand and model as I have now, so I replaced it with no problems.

So here's the problem I'm having now.
The system was running fine then all of a sudden it powers down on me while I'm troubleshooting a wireless router issue, and it wont power back up. I Unplug the PSU, then I plug it back in and hit the power button. The system powers on for 2-3 seconds, fans kick on, HDDs spin, then powers back down. This is all I can get it to do.

I stripped it down piece by piece, repeating the power up/power down issue, until I was eventually working with just the PSU, MOBO, Memory, and CPU. I swapped the PSU with an older one and had the exact same effect. I then even took out the CPU and had the same thing happen with just the PSU, MOBO, and RAM plugged in.

The System will always just power up for 2-3 seconds, then power down.

The only way I can get it to stay on longer than that is if I remove the memory. Then it will stay on beeping at me because of the memory error.

So again if anyone has any ideas, I would love to hear them. I don't think it's a PSU problem and I am leaning towards a dead CPU, but really don't want to order the part only to find out I was wrong and it's a MOBO issue.

Thanks for the help!