Okay, I think that I have some insight on this problem. I have a mac pro dual quad 2.8. It is situated on extra large old executive desk with the mac pro rotated sideways to give me lots of room to work.
I started noticing the same problem when i close the desk drawer too quickly. The drawer opens and closes directly beneath the computer. I did basic troubleshooting, unplugged everything and used wall outlets instead of going through a power strip. Nothing worked, if i closed the drawer in the right way, it would reboot or freeze every time.
Here is where it gets interesting. When moving everything around, I unplugged my lamp and noticed that a bright red light would illuminate at the back of the case to the left of the IO panel. Googling this brought up that the lights actually indicate memory status in the riser ram cards.
If you open the case from the access door you can see just how far these riser cards stick out. They nearly touch the side of the door.
I believe that the opening and closing of the desk drawer causes a vibration of the access door which in turn bangs on the memory riser cards, causing lockups and complete failures.
The new nehalem based mac pros have a totally different design for the memory section, incorporating another circuit board alongside the bottom of the case, just above the power supply. I believe that the memory risers were deleted due to this problem.