Following on from Nino's comments above, grab hold of the lamp matrix connector(s) during gameplay/gameover mode, and give them a wiggle, you'll even see the arc'ing sparks within the connector to tell you exactly which column/row has a dodgy contact. (assuming it's not a driver transistor issue).
I've seen this a LOT in many old pins, it's now become one of the 1st things i do when i find a pin with a dodgy lamp/switch matrix....
Hope this helps too.
MM.
