I have made some progress over the last couple of days.
I looked at the power rails before and after the hang and nothing to report there really. At bit of hum (about 40mv) but it says constant.
I found that IC 69, the 6522 was getting too hot to touch. Cooling it kept the machine running a little longer. I looked at the circuit diagram and IC69 is connected to the outside world via an 8 bit buffer. I pulled the buffer and the temperature went right down. Looking at the buffer, its pins were filthy and so I cleaned them up with a pencil eraser and then some IPA. When I put the chip back in the 6522 stayed cool. Result.
I think the buffer wasn’t getting powered properly because of the dirty pins and so was taking “Phantom” power through it’s I/O lines. I’ve seem something similar before.
Now the machine is running for a few minutes before hanging. Much improved.
My plan now is to work through the rest of the chips making sure that they are not just seated well but also cleaned