Quoted from snyper2099:
This is a VERY common Bally/Stern display problem. It is actually caused by one bad display affecting how the other displays show digits. You will need to unplug each display one at a time to determine which one is causing the issue. In other words, the display showing wrong data is not necessarily the faulty display.
Yes, try swapping displays as well: If the problem does not move with the display, then it is likely a flakey spring-clip on the female-side of the connector. I have found these by pulling each wire-spring one-by-one on the problematic connector, and the bad ones will usually be quite obvious (either broken completely, or with almost no spring left)