Just a quick update, for anyone chancing upon the thread with similar issues. Unfortunately I have no revelations or even useful advice! :’(
Whilst addressing an entirely different issue on my parents’ machine I put a pci IDE controller card in to my machine, to test it. POST… Then POP! Dead! Nothing, NADA. Sigh! New mainboard time. Eventually found a fleabay bargain and installed it. Only to find that there was nothing wrong with the original and that it was the PSU that had died. Currently running with an old, no-name, cheapo, underpowered, PSU that I found in the shed, lashed up using only the 20 pins power line to the 24 pin socket , connected to the “new” mainboard.
And the random freeze-ups, hangs, scrolling issues etc etc have disappeared. I know that this is not terribly useful to anyone, but it IS possible that the old (very high quality, German engineered, equipped with "Over Voltage Protection ™ Over Current Protection, Over Temperature Protection, Over Power Protection, Short Circuit Protection, etc etc etc.) was on the verge of failure, and the install of the card was entirely coincidental, and that the PSU was in fact the original cause of the hangs etc. I doubt it but it is possible. Of course the other variable is the mainboard, but I doubt that too!