It looks like we have a faulty Screen, or at least one that is more susceptible to interference than the others. On one occasion the screen did not display anything but otherwise is appears to be OK at first, but after a few minutes operation the image begins to flicker and this flickering gets progressively worse. The other three Screens appear to be OK, including when plugged in to the same position.
Whatever it is that is failing would appear to be related to suppression of EMI. When I shut the Pi down, the interference persists until the Pi has completed all activity. At this point the Pi still has power applied, but the Monitor has gone into shutdown because it has lost the signal from the Pi. I believe that I have confirmed this in principle by booting the Pi with the Monitor and keyboard disconnected. I then started the software using SSH and there was no sign of flickering for a considerable period (at least 10 minutes). Flickering then started; initially as an odd extra line and then gradually it got worse. This time, when I shut down the Pi the flickering persisted until I turned off the power. I think that this proves that the susceptibility increases steadily after power is applied, so at first it is negligible and then it picks up high amplitude interference from the Monitor (possibly the adaptor which is quite close), but eventually it reacts to interference from the Pi itself.
Penri,
How many OLED Displays did you buy? I thought you said seven, but I can only account for six at the moment. I have five here and I gave one back to you a while back, Should there be another?
I still cannot get the Screen at Position 4 to work, so that's my next job.