I’m putting this here, with the solution. in case anyone else has the same problem. The OLED screen image was periodically jumping and jumbled. I can’t upload the video here - says new users can’t upload - so see it here OLED misbehaving
I had been trying out the various settings of the pironman app. This behaviour persisted even after a reboot.
The solution is simple:
sudo systemctl restart pironman5.service
Found this in the answer to another question about a different type of OLED problem.