Thanks, all. We've also been through Tom's pages with no results. The conclusion that it is a 'timer' is ours too, but we cannot find anything that is
a)Set for 30 minutes
b)Will shut down the pc
The board is around a year old, and the problem started when the machine was moved out for redecoration. We have tried with most bits removed, but RAM yet to try. Firmware has not been changed. Initial suspicion was the mobo but the accurate 30 minutes cycle has cast doubt on that. On-board NIC has 'disappeared' as well and bizarrely the sections of the mobo install DVD which install NIC drivers do not appear in the 'menu' now.
Weird!