I have a Top Score (2 player version) and have been working off and on for a while now on the same or similar problems. First issue I had was the bonus balls were being kicked back and double counting bonuses or not counting them at all and being left for the next player/ball. I found that there is a wire catch that was stuck up and wasn't preventing balls from being kicked up the channel. The next problem was bonus balls were not being added (random bonus kick out) or subtracted (drain and add bonus kick out). I traced it back to the add bonus delay relay. A normally closed contact that isn't releasing. I replaced the capacitor in the add bonus delay circuit which didn't help. This is on a removable circuit board with a diode. If I remove it (circuit board) the machine will play fine except there's no delay which only comes into play every now and then. You may want to try that. Also clean the add bonus relay contact and the 500 point/add bonus relay contacts. I just purchased some jumper wires so I can hook my meter up to the add bonus delay relay and check solenoid voltage during play through the coin door.

On a side note, here is a pic of the bonus ball assembly.176696.jpg