I don't have my 630 in front of me so I don't know if it has one but some Macs have a locking switch on the back that automatically restarts the machine in the event of a power failure. I don't know if your problem is related to this but you can look on the back and if you have a button with a slot like a flat screwdriver might fit into, try turning it gently until the button pops out and see if that fixes it. It shouldn't be restarting from a normal shutdown, though. Only if there is an interruption of power while the machine is turned on.