Here's a real doosie; I may just give my firstborn child to whomever helps me solve this problem.

In July 2008, I assembled what would be my desktop computer for graduate school. Here are the specs of the machine I built:

• Thermaltake 750W PSU
• Corsair Dominator 2x2GB 240-pin SDRAM
• Thermaltake Tower
• Asus P5K Deluxe Motherboard
• Intel Core 2 Quad Q9300 2.5GHz CPU
• 2 x GeForce 8600 GT
• WD Caviar Blue 640GB hard drive
• CD burner
• DVD burner

Soon thereafter, I ordered a new motherboard (because I was an idiot; that first motherboard supported CrossFire, not SLI), an Asus P5N-D. I was originally running Windows XP SP3.

Pretty much right into the start of the fall semester, my desktop would simply lock up after awhile. If my system was largely idling, it would be after 1-3 days. If was gaming, it often happened an hour or two into my gaming session, indicating a link to activity level. Here's where it started getting interesting.

• I started looking at the system temps. The CPU was warmer than it should have been (~60s C), so I purchased some more efficient cooling compound a way better cooler for it. Now it hardly goes over 40 C. Intel was even kind enough to swap it out for free, just to rule it out. Lockups continued.
• The graphics cards were also running pretty warm: about 60 C idling. Removing one of them seemed to improve stability a little bit...as in, it wouldn't lock up quite as frequently, but still always eventually locked up. But it didn't matter which card I used or removed, the lockups continued.
• I reverted back to the original motherboard, the P5K Deluxe. Lockups continued.
• I purchased an entirely new motherboard, eVGA's nForce 750i. Lockups continued.
• Ran memtest86+ over and over and over, with no errors. Even RMA'd the memory. Lockups continued.
• Replaced the PSU with a Corsair 750W PSU. Lockups continued.
• Tried disconnecting all IDE drives (HDDs are SATA). Lockups continued.
• Replaced both graphics cards with a single Radeon HD 4980. Average temps are now always around 50 C when idling, 60 C only when gaming. Lockups continued.
• Throughout the whole ordeal, the system has been upgraded from Windows XP SP3 to Vista 32-bit, to Vista 64-bit, and is now at Windows 7 64-bit. Lockups have occurred at every step along the way (each OS was in place for at least a few months before the next upgrade). Edit: By "upgrade" I mean clean install each time. In addition to those reformats, I have performed many, many other reformats of the system and a reinstall of whatever OS had been previously installed in an attempt to rectify this problem, to no avail./Edit

When the system locks up, there's no blue screen, no reboot, no error message of any kind. It simply freezes in place until I hit the reset button. Very, very rarely, once Windows boots back up, the system informs me that Windows has recovered from an error, but it can never find the source aside from some piece of hardware. I've swapped out every component in this computer, and there are more fans in it than I care to count...though for the sake of completeness:

• top 80mm case fan (out)
• rear 80mm case fan (out)
• rear 120mm case fan (out)
• front 120mm case fan (in)
• side 250mm case fan (in)
• giant CPU fan
• on-board motherboard fan (the eVGA board)
• triple-fan memory setup (came with the memory)
• PSU internal fan
• another 120mm fan I stuck on the underside of the video card to keep hot air from collecting at the bottom of the case

I'm truly out of ideas. ANY help at all would be oh-so-very GREATLY appreciated. Thank you!