New
#51
Solution has been found
Having stripped down the PC and thoroughly cleaning the computer, I can report that it is running reliably and the only reboot that happened was 10 minutes after I started the PC. It must have coughed out the last bit of dust because it has run beautifully since (which was 3 weeks ago), so I consider it all clear and give it a green light.
Thanks to all those who responded to this thread, especially Dalchina, much appreciated.