OLED and Pydpiper - display missing lines

Has anyone had this problem?

Using Pydpiper my little OLED display was perfect. However, whilst trying it out with some long leads in my camper van, I noticed that there were lines missing from the display. Being fairly sure that I had not shorted/wrongly connected anything I assumed that the long run of leads was my problem.
Back to the indoors system, with normal leads, the missing lines were still missing.
I tried a different display, same problem. I tried a couple of different Pi, problem persists.
I’ve re flashed new cards, same problem.
I’ve tried using the Pydpiper plugin rather than installing Pydpiper from source - still missing lines.

Can anyone shed any light on my missing lines and turn D D King back into B B King?
Many thanks in advance for any help.