Update in case anyone is interested/cares
It is the RAM. I had more than a week of no problems, despite not having changed anything. Then I finally had a failure to boot properly (though not a failure to POST). I ran memtest86+ straight away, and it errored. I then tried a stick at a time, but found no errors. Of course, by then, everything was warm.
I then swapped the RAM between the two machines.
I’ve had over a week of no failures, but today the other machine (now with the same RAM) failed to boot properly. Again, memtest86+ showed errors, but attempts to isolate it to a single stick didn’t get anywhere.
Luckily the RAM has a lifetime warranty…