Thanks for the tip, 18-Wheeler. I was wondering how accurate the temperature sensors were... Tom's Hardware seemed to imply that some of them were very accurate, but it sounds like it depends a bit too much on the BIOS (calibration issues??).
Anyway, the intake fan (see above) didn't make a lot of difference - the 100% load CPU temp now down to 64C or so, although the motherboard temperature is now only around 28C, with ambient temp around 22C. The PSU runs *much* cooler, which is good - I wasn't happy about the amount of heat that was building up in there.
18-Wheeler - is the ASUS overheat protection meant to trigger a controlled shutdown of Windows, followed by a power-off? If so, it certainly didn't work properly in my case, for whatever reason - the thing just locked solid. Hmmm...
cbl.