I disabled windows update (it checks for updates but does not download or install without my permission), and although I've only restarted once since then, I had no issues. So whatever the problem is, windows update was certainly a component of it, confirming the gut reaction I had the very first time this happened. That helps me figure out what the problem is and lets me use my computer for now, but I don't feel like it's a permanent solution. I'd still like to know what the hell is up.