I had a very similar issue with my 7 digit display in a Skateball. Both commas would light when the scoring showed "00" before gameplay started. It ended up being a long process of troubleshooting, but in the end it was the zener diode VR1.
You may want to check the voltages on both sides of this diode - there should be about 100-110 volt difference. In my case, there was no difference in voltage - the zener was shorted. Replacing VR1 solved the problem.