This Is Not A
I am - for reasons to do with the possible intercession of the three true gods, Eris, Murphy and Sod - somewhat reluctant to go into the chicken-counting trade just at the moment, but I think the problems I've been having with this PC for a week and a half have resolved themselves.
Moreover, I think I know what might have caused them.
First, read this.
What my attempt to prevent a recurrence of that little upscrew actually meant of course was that - with the exception of the occasional power cut and the two or three times that I've disassembled the whole set-up in order to clean - the PSU has had power going to it continuously for over eight months. This has obviously caused it to lose its stability and - to use a technical term - throw a wobbler.
Since last Tuesday night, once I have powered down the PC I have also powered it off at the surge protector and flipped the rocker switch at the back of the tower to its 'Off' position.
Since then, no problems.