I have asked for clues on the StarDot forums, a valuable source of BBC knowledge. Within hours some very good ideas were suggested.
So far I have tried cleaning and re-seating the socketed chips and swapping the 6522s but to no avail. I have measured the volages on the supply rails before and after the hanging but they don’t change.
I also tried putting a spot of IPA on the back of the chips to see which was getting hot. The hottest should evaporate the IPA quickest. I couldn’t tell but I thought the machine stayed running for longer because of the cooling effect but I’m not sure.
I’ll keep trying.