Normally this type of erratic behavior is caused by one or more misbehaving installed program(s).

I don't like to use names or point fingers but I've had problems with Norton causing this type of problem more often than not.

 

If you have a defragmenting program that runs in the background and an anti-virus program installed that monitors disk activity, then sometimes they get into a quarrel when they both attempt to access the drive causing a reboot. This can be an endless cycle until the computer is powered off.