Inspiron 580 Random Reboots

So the title pretty much says it: quite randomly, the computer will freeze for a couple of seconds, and then reboot itself. The 'repair options' screen displays on the next startup. I always ignore it and continue to boot. I have actually been working on this machine for months. So I tried to fill out as much hardware info as possible in the registration form, so it should be viewable on my user account page. It's a Dell Inspiron 580 running Windows 7 Pro SP1 x64. I have professional Avast AV and firewall software running on this machine, as well as Malwarebyte's pro scanner (for flash scans). I have been up this machine's a$$ inside and out looking for malware, locking down IE, the firewall, installing security updates, etc, and I am convinced this is not a virus of any kind. I even threw on an IP firewall, fired up Wireshark, and let it go for an hour looking for FTP/SMTP logins or other suspicious activity, but it was all quiet.