For a while now, the power supply has been heating up quite a bit. It seemed as though insufficient air was being pulled through the power supply by its fan.
It turns out the the problem was the positioning of the PSU and its fan relative to the heatsink of the CPU. The fan was positioned exactly level with the heatsink at a distance of about 12mm. Instead of air being sucked into the PSU and out its vents at the other end, I believe this was causing air to circulate between the middle of the fan and the outside of the fan; needless to say, it was getting fairly hot inside there.
I've now moved the power supply about half a fan's width further along, and the air flow seems a lot better. When I lengthen the wires from the PSU to the motherboard, I'll move it even further away.