>> (Inconsistent symptoms, but machine always locks up within 4 minutes of
>> cold
>> power up, regardless of which operating system is being booted.
>> In fact, on one occasion it locked up in the middle of entering the
>> BIOS
>> password. Commencement of problem coincided with starting to use
>> broadband
>> (connected to router by ethernet cable). Tried replacing use of Realtek
>> ethernet
>> controller with Intel Gigabit PCI ethernet card. No problem if keep PC
>> switched on permanently)

I would suspect a faulty PSU, or failing capacitors on your mainboard.