EDIT: Nevermind, most likely a hardware issue, something overheating (from playing silly Crush The Castle: Tower Defense earlier today, most likely). I haven't had a problem like this in years, but it isn't a first for me. Repeated tests made it look like it's having a hard time basically loading ANYTHING after a few operations have been performed, leading it to overheating and erratic behavior. It took AGES to load the game, but then it ran perfectly.
Gave it 6 hours of rest, and I probably won't start working on it again till tomorrow.