Thanks to both Gerry and Bob for your replays.
Bob - I couldn't make the link to the XP optimisation guide work, but I googled and found
http://www.machsupport.com/forum/index.php?topic=6346.0;wap2 which looks like the same thing. I applied all of the recommendations and the system works fine now. Pulse rate is stable and the driver test program is now saying all is OK.
Gerry - Your suggestion of changing from ACPI to standard mode was contained within the various changes in the link I found.
Back up and running now and in production again. I am not sure which of the steps in the process was the critical one but it has certainly worked.
Many thanks again to you both.
Tony