My 2.2 GHz MB suddenly began to restart when shut down, and it will not sleep. I have narrowed the problem to hardware, but I cannot specifically identify what is causing it.

Console does not always state the same thing when checking to see why it wakes from sleep. Sometimes it will read Kernel Wake Reason = PCI, but other times it will only read Kernel Wake Reason =

There is nothing attached to the MB. It is a standard model from MacMall with no upgrades other than RAM and the hard drive.

Someone has suggested that this is caused by corrupt firmware. Can this be the culprit? Is there any way to troubleshoot further if necessary?

The MB has been tested with two drives that have different OSes, the power management files have been deleted, the PRAM has been zapped, and the power button was held after removing the battery. No luck with any of these approaches, so I lean toward a logic board issue.