Almost sounds like a heat issue.
Make sure it has good ventillation during operations, as well as noting what is running when the event takes place. Things like number of programs running, duration of continuous operation until the reboot occurs.
Also, perform and error check on your hard drive:
- Open My Computer> highlite and right click on your C drive
- Select Properties
- Select Tools tab
- Click on the Check Now box in the Error -checking section
- Check both boxes and then click on Start
- This will schedule the Error Checking upon reboot
- Reboot the system and allow it to run