surge ?

oops thats what you meant by 98 first - heh !

I believe that the auto update should have taken care of it. I did exactly the same with WinME and there is a noticeable slowing of the shut down, so I know it was uploaded and installed in auto update for this OS, probably the same for yours but I cant say for sure.

re :- "Also have you double checked the mobo manual against the leads and swapped out switch leads just in case ? And the mains power supply lead"

means - have you used alternative leads which you know are functioning for such things as the main power lead and the motherboard case switch, and have you checked for the latter that it is connected to the correct pins with the correct polarity as instructed in the motherboard manual and not accidentally misaligned or reversed (some mobos have a forest of pins which its easy to make a mistake with)

I am thinking that a loose wire somewhere would cause the random shutdowns you are experiencing.

But equally if you have brownouts and dodgy mains electricity, it may be that you are getting bad current surges causing fluctuations in mobo voltages triggering protective shutdown, in which case do you have a surge protector ?

What is the house wiring like & the meters ? are there any heavy electrical appliances being used locally which might cause voltage spikes ?