Restarts are usually caused by heat issues. A common issue with desktops is the cheap OEM heat paste on the CPU has turned to powder, and is no longer working. So the CPU overheats. You need new quality paste (Noctua), and replace it.
But there's a lot of issues to troubleshoot if that's not it.
I know it's been 2 months since your post, so what was it? Or did you not get it solved yet?