Random Reboot no bsod

long time reader, first time posting - unfortunately with a major headache.

Current System Specs:
Intel Core 2 Duo E6700 Conroe 2.66GHz (stock cooling)
EVGA 256-P2-N583-AR GeForce 7900GT x2
CORSAIR XMS2 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) TWIN2X2048-6400C4
ASRock N7AD-SLI LGA 775 NVIDIA nForce 740i SLI ATX
XCLIO GREATPOWER X14S4P3 550W ATX12V SLI Certified

The Problem:
Random restarts, in windows, at the bios screen, running memtest86, during and after boot, all without any warning or a bluescreen. The restarts are completely random in time, and they can occur during idle or full use and sometimes one instantly right after another.

What I've tried:
I've stripped the rig down to just 1 vid card, 1 ram stick, the processor, and a cd-rom drive running memtest86 and it still restarts randomly (with zero errors in memtest).

I've tried alternating the ram/video cards and their configurations to try to eliminate a piece of hardware with no luck.

I have already replaced my motherboard from a ASUS P5N32-SLI SE Deluxe to the ASrock board, with no luck. (the previous board eventually would not post, even with all parts pulled)

The rig also originally, used a Antec NeoPower 550W which would not power on after being pulled from the system. (I made sure green return line was set).

I've verified the voltage of the new PSU with a multimeter, in the bios, and with HWmonitor (cpuid's hardware monitor). I used the same program to monitor temperatures, while running cpu burnin and other stress testing software. All temps stayed within acceptable limits.

While running CPU-Z I've noticed the ram timings that the bios has picked are 5-6-6-18, which vary from the JEDEC standard 5-5-5-18 at 1.9v ... I've tried setting the proper timings manually in the bios with no luck. Just for kicks i thought I'd try 4-4-4-12 @ 2.1v (as stated on newegg and corsair's site) the mobo would not pass post - a Solid continuous BEEP - until I reverted the timings.

I've also tried manually setting voltages with no luck there as well.

For giggles I used artic silver 5 underneath the stock cpu heatsink/fan to see if it would make a difference, no luck there either.

In desperation I also tried reformatting - no luck, Xp sp3- I've unchecked the bsod automatically restart option.


Where I stand?
At this point I'm down to processor/ram/videocards as the culprit; however, with zero bsod errors I have no clue as to the cause.

Any help/suggestions would be greatly appreciated.