I have Zodiac, which is the same thing as Planets except that Planets (the add-a-ball) is even more rare! We like ours and play it a lot.
Switch #2 on the score motor, a switch on the Tilt Relay, and a switch on the Outhole Relay each could make it step from ball to ball if it's adjusted so it can't open.
ZodiacPlanetsBallUnitStepUp (resized).png
The second picture shows the Tilt Circuit - the plumb bob, the ball tilt (next to the plumb bob, the smaller rolling ball that keeps you from lifting up the front of the game) and a slam tilt switch could all be making it tilt and move to the next ball. Does the "Tilt" light on the backglass flash on and off as it cycles through the balls? Let us know what you find!
ZodiacPlanetsTiltCircuit (resized).png