Doby

Do the defrag again and see if it reboots, it could have been random, if it does it again go to the hardrive manufactures website and download there tools to check the drive.

I think you have already done this but just for the heck of it do it again.

Still if you can barrow a psu just to try that would be best because even though the voltage read good I have seen them just drop the voltages for a second that the monitoring software cannot detect and cause a rebbot