PC shutdown then restart by himself. How can i diagnose

[arjfca]

Hello

I bought a used PC as a backup for my kids when they come home. Seems to work fine but one in a while will shutdown by himself and restart

PC is an HP Pavilion a6318

- I did pass the memtest with no fault for a complete day
- I did check the inside, there is no apparent accumulation of dust
- I did test the hard disk and no fault found
- Fans are running. No evidence of overheating
- Did a Virus and Adaware test

What other test could I execute

Any help appreciated

 

[Kaktussoft]

disable autorestart on crash
Disable Windows 7 Auto Restart When System Crash - Windows, System, Settings, Advanced, This, Uncheck - Zhacks

 

[essenbe]

If your computer is still under warranty, I would contact HP support. I don't know if you have made any changes in BIOS, but go into BIOS and load optimized defaults, save and exit and see if that helps. Other tests you can run are prime95 to test the CPU or Furmark to test the Gpu. You can also run checkdisk. I would suggest running sfc /scannow to verify and repair any system files. If all of those tests pass with no problems found, it is likely it is a poblem with the motherboard/BIOS or a PSU.