[Help] How do I diagnose cause of freezes/restarts?

My machine is running great -- except that once or twice a day it just spontaneously restarts. 90% of the time this happens when I'm away from it and leave it running for a couple of hours.

I thought it might be because of sleep issues, but I set Sys Prefs Energy Saver to "never sleep" for both machine and screen; only screensaver runs when idle. This did not help.

I thought it might be because of overclocking, so I tried dialing my OC way down, but this did not help either. (FWIW, not sure if related, but all my BIOS saved presets disappeared after one reboot.)

It also happened once or twice while I was working ... it wasn't so much an instant restart, but more of a "freeze" where everything, including mouse, just stopped responding. I'm pretty sure it stays in that frozen state for a few minutes, then reboots itself.

What steps should I take to diagnose? Console > system.log never shows anything unusual prior to the crashes.

(Here is my config.plist)

(mobo gigabyte x58a ud3r, cpu i7 970, os sierra 10.12.6)

thanks!