I just built a new computer and installed Windows 8.1. Everything is fine, except every once in a while Windows will just lock up, freeze for about 30 secs or so, then reboot. It happens when I go to switch applications (once just by hitting the Windows button), and when there is a lot of disk activity (downloading from the net or Steam, or installing a program). I don't remember it every rebooting without a download/installation going on (but I am still setting it up, so there is a lot of downloading/installing going on). If I stay within an application, I haven't had any problems yet - and that includes running benchmarks like Star Swarm.

I thought it sounded like it might have been a memory issue, so I ran memtest86, but every test passed.

Anybody have any ideas what to check?

Thanks!