Well, there are many things that can cause this, but in my experience a lot of display-weirdness issues are due to connector issues, generally broken or corroded spring-clips in the female-side (I'm going to assume you've already tried a display swap to exclude that you're not talking cold-solders on the male-side). The problem with the "Display Test is OK" diagnosis is that all displays are being driven by the same digits at the same time, and some problems only show up when each display is driven separately as in Attract Mode or in-Game mode.

Try popping out the wires from the credit-display connector one-by-one (you can just use a small jeweller's screw-driver to push the tiny spring-clip down and pull the wire out): You may find one or more bad ones that way.