I've had a similar experience with the ribbon cable connector (data lines).
I was asked to look at a DMC (dot matrix controller) exhibiting page errors. Sure enough display test shows page errors. I took the board home.
Put the board on my bench. Passes display test perfectly. Weird. Put the board in a machine and it passes the display test perfectly. Put the board back on my bench. Passes display test perfectly.
So I give the board back to the owner and well ... it works perfectly.
I call it "pinball voodoo".