A friend was having computer power cutouts - possibly the power supply - but I suggested checking temperatures first. So we both installed Speedfan and goodness, it was my machine that was overheating!
So off with the cover and in with the vacuum cleaner. Success! The CPU fan is now speeding up properly.
Speedfan can also adjust speeds and do other geeky stuff, so I tweaked to slow speeds when not busy.
[added] ... and in my friend's case it was dog hair!
