Hi there,
I am also experiencing exactly the same issue. It used to be quite frequent, but it hasn't happened now for about two weeks. It seems to be utterly random. Large or small load, cool or hot, long or short uptime... doesn't seem to make a difference.
I have the 2.66 model with 4GB RAM (all Apple original in 512's), ATI 1900XT, a WD 500 in bay 1 (Tiger), 2 WD 250's (Striped homes) in bays 2 and 3, and a Seagate 250 for other OS's (Vista Ultimate 64 and Ubuntu Edgy).
I have put the config back to how it came out of the box, re-installed the OS, reset the SMC and PRAM many times, but that has made no difference, it still happens occasionally in either configuration.
I have also experienced the same problem when using Linux booted in native mode. A click and then it reboots as if someone pulled the power. As it happened whilst booted in a different OS I am now assuming this is a firmware/hardware failure of some description somewhere along the line. As other users sometimes there are 2 or 3 reboots before it makes it back into the OS.
I have run all kinds of diagnostics programs and they all report no issues with anything. The only other problem I have observed is display corruption when using Aperture, but I have put this down to poor GPU usage, or maybe not...
My machine and display (20" ACD) are plugged in to a Belkin 1500VA UPS.
http://catalog.belkin.com/IWCatProductPage.process?Product_Id=186894
The UPS never complains about the power coming in and doesn't appear to have anything to do with these resets. It's stable and never complains.
I believe there is a faulty component in the Mac Pro power supply that is causing this, but I do not have enough knowledge or equipment to prove this. I have also suspected the graphics card on occasion but again, I have no evidence. I have spoken to Apple support about this, but even with the £200 Apple Care they said I would be without the machine for up to 2 weeks which is unacceptable to me as I need it to earn a living.
I have been living with the random reboots for now, but it has lost me several hours of work before now so it's becoming extremely disappointing and frustrating, especially having now spent in excess of £2500 on this machine. I expect better from Apple for the money.
I am about to spend a large amount of cash buying several new multicore intel workstations for my place of work. I am currently put right off the Mac Pro because of this unresolved issue, which is a shame. It's currently the only machine which will support all the OS's we need to use (Tiger, Vista and Ubuntu).
http://www.flickr.com/photos/danalderman/402482843/
Any info on this would be greatly appreciated.
Kind regards,
Dan.