I haven't done enough testing to nail down the problem yet - if it ever happens again, I'll re-post here with whatever state my computer was in. Just happy to have a reasonably stable system again.
Well, it happened again today. I logged in and within about a minute, the entire system froze. I have not changed anything since last time. Really, I browse the web with Chrome and play some Steam/Origin games. I don't think I've even installed anything new since my last post.
I also have not discovered a reliable "fix." Every time I rebooted, the system crashed about 1 minute after logging in. So, I mucked with things: Reenabled Bluetooth in the BIOS, Disabled Bluetooth in the BIOS. Finally, disabling both Bluetooth and Wired LAN and disconnecting my mouse (no idea which was successful) allowed me to boot without a crash - it's been 5 minutes. Also, several times in there the Windows Recovery screen popped up, because I have to do a hard reset each time. I can't "Refresh" since I don't have Windows 8.1 media (?! It was an update from the App Store) and none of the other options were helpful.
So, now I'm running without LAN and Bluetooth and a Mouse. I'll post this and plug the mouse back in - I assume it will work, it has never been the culprit before.
This seems to start happening every 10-12 days, based on my previous posts. This leads me to believe that there is some background Windows process that is causing the issue - maybe checking the servers to reauthenticate my Live Password? And my wireless card is being used before a proper driver loads? No idea.
The fix still seems to be: "Mess around with networking related BIOS options and pray."