Freeze & reboot, but no BSOD

[ComputerClinic]

It certainly sounds like a hardware problem, but that may not be the case. To be positive that its hardware, boot with UBCD and run benchmarks overnight and see if it reboots. Maybe while it is doing this, you could remove the HDD and test it for problems at your office?

I say this because if its a hard drive problem, running benchmarks in UBCD won't trigger a reset, because the HDD is not needed to run the OS.

[Tweak]

You've tested a good bit of hardware so making sure which it is as suggested above and as I was pointing to seems the only options left, seems about certain you'll find the drive or even a cable that is the culprit from the sound of it.