I will hopefully get this checked tomorrow. Thanks v much for keeping me
informed.
Nick

I've had my session with the problem PC and this is what I found:
When the PC shut itself down the Vsb remained normal and the PON pin was
low
indicating that the PSU was not shut down by the power control flip-flop.
In
fact holding the power button for 5 secs reset it to the OFF state and
pressing it again powered up the PC as normal. So the PSU shut itself down
via a different mechanism. Inserting a CD in the troublesome CDRW drive
revealed why. When it spun the disc it sounded like a jet engine even
vibrating the desk itself. The 12V line dropped to 9V indicating a large
current drain and the PSU shut down about half a second later. So an
overload on one of the supply lines can cause the PSU to switch off.
Changed
the CDRW (Ionics - a Philips subsidiary) for a Samsung and all is now
well.

If you have an access to a multimeter I suggest you check pin 9 (Vsb:
+5V)
and pin 14 (PON should be close to 0 when the PC is ON and close to Vsb
when
OFF).