Thanks for responding, sking.
I have only ever had one instance of the game running at any given time. The problem has happened multiple times, and I haven't been able to figure out why.
Recently started a new game, and if I experience the same problem, I'll try uninstall/reinstall, I guess. I'm kind of grasping at straws.
Often, when I reenter an area I've previously cleared out, the area has darkened again, and monsters, traps, and treasure have returned. I have now defeated Maerwald twice, for instance, and while it's nice to grab the same loot twice, it's kind of slowing down the game.
Any ideas for how to solve this?