Could be a lose wire on the coin unit, the player reset does not work without a lead from it.
That could explain the behaviour.
So: check the wires making contact on the two players on the coin unit.

Could be a too close contact on the score motor that releases the relay after the motorturn, or a nearly closed contact on the relay (closest to the relay, hold contact), or a bad spring on the relay.
Top contact on the sixth set on the motor, the one just before the score cams (last two).