It would help if you provided us with more information regarding the make and model of this computer and the operating system it is running.
How are you monitoring your temperatures?
Since you mention a PSU I'm assuming that this is a desktop? When was the last time that you cleaned the inside of the case?
Since this computer is over five years old I'm guessing that you are running Windows XP?
If this is the case, then let's stop the automatic restarts.Start> right click on My Computer> Properties> Advanced> Startup and Recovery.
Under startup and Recovery click on Settings, uncheck the box in Automatically restart under System failure.
The next time this problem occurs you will get an error message. If the error message looks similar to the image below there will be a Stop error, it will be similar to the one at the top of the page in the image below ***Stop: 0x000000d1, or something similar. Please include this in your next post.
If this isn't the same type of error, please post what you are seeing as accurately as possible.