There are a couple of other "faults" which may be completely unrelated, but which I'll mention here in case they help the diagnosis:
1. The motherboard's software (Asus) occasionally reports absurd temperature and voltage alerts - things like -250 degrees, or 0.1v. We looked into this, and it seems to relate to the Asus software rather than being genuine temperature or voltage faults. If the temperatures or voltages had fluctuated as wildly as they have been reported, then there would have been other problems, and in every other respect the computer has been rock solid.
2. The front panel power switch has been playing up the last few months. Most of the time it works fine, but occasionally it won't power on when you push the button - you have to wait anything up to 10 seconds before it turns on. We put this down to being an intermittent fault in the microswitch.