Computer might shut down if motherboard is bitching - for example, if the controller (the one that connects your hard drive with motherboard) is fucked up. I had the same prob.
It also might shut down because the overheat:
- if the coefficient of your processor's power has been manually speeded up.
- if BIOS settings have been changed somehow.
- if it has no "breathe", like heating itself from inside and has no way for that heat to go out.
- if hardware is dirty (dust) - as dust is a thermal conductor, - clean the contact-cables that connect cooler and your processor. I had this prob too.