Just to close this out - It's now one month later and for the last two weeks my old Glo HD has been behaving normally again.
I have not made any additional changes since swapping out the SD card and updating to FW ver 4.26.16704 back at the end of July.
The behaviour changed back to 'normal' in the middle of a book, and so it wasn't an issue with a particular file (and I'd tried a bunch of different ones anyway).
So I have no idea what caused the issue, and no idea what fixed the issue

but I'm very happy to have it back, and thanks again to everyone who suggested things to try.