Locking up, spontaneous reboots, after clean install

My friend's Dell desktop was locking up for about 2 or 3 minutes, then working again. Or it would reboot spontaneously. I ran chkdsk /r, it found only minor errors in phase 4, (file system errors). I ran Windows Memory Diagnostic, 8 passes, no errors. I did a clean reinstall of Win 8.1, the same behavior persists. Any ideas where to look next?

Thanks

Hello Vincenzo,

You already done a clean install which format your hard drive and the problem still persists, it may be a hardware problem.
Try check your hardware for issue like hard drive failing, or bring your friend's laptop to a computer shop to have the professional to check for you.

Cheer!