Computer keep restarting on it's own. Looking for troubleshoot service.

My desktop keeps restarting on it's own.

Sometimes it can last me quite awhile, after a half/an hour usage it will restart.

Once the restart kicks in, it will keep restarting after the log in. Its a cycle loop. log in restart, log in restart.

Only after i off it for sometime then it can last me another half/an hour again.

I also realised if i watch videos, some files will restart immediately.

I check my temperature and fan, all is fine.

Anyway provide services for troubleshooting? Or where can I go for help.

Im intending to upgrade anyway.