1992 Toyota Corolla Engine running too hot

I'm having problems with the engine running hotter than usual, straying dangerously close to the red zone. It overheated once last week.

I've already checked the coolant level, which was slightly low, but not dangerously so, and topped it back up. I also added a bit more oil to bring the level back right to full. This doesn't seem to have helped.

As far as I can tell, the fan is working properly. I don't know what else to check for. Suggestions?