For anyone having this issue :
Shut down.
Pull out both RAM drawers.
Remove all RAM.
Remove all dust (preferably by dry compressed air).
Carefully but firmly re-seat the RAM on the drawers.
Re-seat the RAM-drawers in your Mac Pro.
I've had this random shut-down issue right around the time Steve died, so it could also be related to his reality distortion field energy playing havoc on my Mac Pro ... but somehow I doubt it ;-)
(The restart occurs after an unscheduled shutdown, if you have automatic restarts enabled in the Energy Saver preference panel.)