I think I may have inadvertently re-seated the RAM modules when I opened it back up to clean it again and then set the PC itself horizontal rather than vertical, which it had always been! It’s been working fine since then.
I have digital manuals I found online for my car, AC, dehumidifier, microwave and many other things yet it never occurred to me to find an online manual for this old PC. So thanks again for the very valuable ‘RTFM’ advice and for actually finding the manual for me!