Before adjusting the travel of various stepper arms please verify whether the Left Advance relay fires every time the Left Bonus Unit advances. It could be that the Bonus Unit is working correctly and is just doing what it is being told to do.

The Left Spinner switch fires the Left Spinner relay which advances the small Left Spinner Unit stepper. The Left Spinner Unit stepper has ten steps as shown in the schematic:
Grand Prix Left Advance relay (resized).jpg
When the Left Spinner Unit is in the 6th or 10th position it fires the Left Advance relay which advances the Left Bonus stepper. That gives you the two bonus increments per 10 spinner turns. The 3/5 ball adjustment jack will allow the bonus to advance a third time (at position 3) if it's in the 3 ball position.

I suspect that your Left Advance relay is firing more than it should. If you can verify that it fires every time the Left Bonus advances we can drill down into what could cause that.

BTW the specific game rules and lots of other helpful information is in the game manual on ipdb.org.

/Mark

Great observations and I agree with your assessment. Time to break out your meter. Unplug your game and clip your meter between the yellow wire and the yellow-blue wire in the schematic above on the Left Spinner Unit stepper. Manually advance the stepper and watch the resistance between those two wires. Depending on your game setting (3 or 5 ball) you should find 2 or 3 positions where the resistance through that unit drops to an ohm or less. That tells you where the Left Advance relay would fire if you were playing the game. Convince yourself that you can identify where those 2 or 3 positions are that are working correctly.

Now move your meter to the Right Spinner Unit stepper and clip on to the (same) yellow wire and the white-blue wire. Repeat the experiment and see if you can find the 2 or 3 positions where the resistance through the stepper drops to an ohm or less. If you can't find as many positions as you found on the Left stepper you know that the problem is somewhere between your probes in the Right Spinner Unit stepper.

Have a close look at the manual pages dealing with the Left and Right Spinner Units and compare them with the schematic:
Grand Prix Left Spinner Unit (resized).jpg
One thing I can see from your photo is that you're probing two sides of the Spinner Unit where all the wires you're looking for are on the same side.

If you look closely at the diagrams above you can see that the yellow-blue wire on the left spinner is wired to the copper trace ring around the center, and that the yellow wires get connected to the yellow-blue in only two of the 10 steps as the wiper advances around the contact board. The yellow-brown wire is the optional third connection.

Once you understand the Left Spinner diagram above you should be able to verify that you're getting 2 or 3 connections in 10 Left Spinner steps. Then you should be able to identify the wire colors and connections for the Right Spinner using the diagram in the manual and repeat the experiment. This should tell us whether the Right Spinner Unit is the culprit, or if the problem lies elsewhere.

That would remove the Spinner Units from suspicion as @rufessor suggested. To confirm you should be able to manually close the left and right spinner target switches 10 times each with the game on and see both Bonus Advance units step twice.

And we're still working under the assumption that the Right Bonus advances each time the Right Advance relay fires, right? That would reduce the chances that there's a mechanical issue with the Bonus Advance stepper.

So we're still hunting for a reason that the Right Advance relay periodically misses a pulse. There are just four paths into that relay:
Grand Prix LR Advance relays 3 (resized).jpg
The top path goes through a switch on the Top Eject relay then through another switch in the Change relay. Most of this path seems to be working if the Left Advance relay is working as expected. It's possible that the Make/Break switch on the Change relay isn't working properly on the Right Advance relay side (green-white wire to green-orange wire) so that's worth a closer look.

The next path to the Right Advance relay goes through a switch on the Right Standup target. That shouldn't be a factor here.

The third path goes through the Spinner Unit and you seem to have proven that that too is behaving as it should.

The bottom path is the lock in circuit for the Right Advance relay. It holds the Right Advance relay on once fired until the advance arm on the Right Bonus unit has taken a full step and opens its EOS (End of Stroke) switch. If the EOS switch isn't opening reliably it's possible that the Right Advance relay stays energized longer than it should and might miss a subsequent pulse. It too is worth examining closely.

Hmm. That is odd. The 500 relay is in the DC part of the circuit and should be independent:
Grand Prix 500 relay (resized).jpg
Note however that both the 500 point relay and the Right Advance relay have a switch on the Right Stand Up target. There could be a short between the switches on that target that lets power from the Right Advance relay fire the 500 point relay too. I don't know if that's the root of your original problem but it's probably worth looking in to.

The wire colors on the schematic will confirm whether you're looking at the correct switches. Rather than disconnecting anything you could put a piece of folded paper between the contacts of suspected switches to see if the 500 relay still fires. Try the paper between the contacts of all of the switches leading to the 500 relay shown in reply #21 in various combinations (all, none, standup targets only, 500 relay switch only) to see if you can determine which path is activating the 500 relay when it shouldn't. If you block all three switches and it still fires there's a short elsewhere like on the solder tabs.

Yes, both standup switches connect brown-black to yellow wires. The switch on the 500 relay connects brown-black to red-brown instead of yellow.

Well done. Did that fix all of the issues or just the last one?

The mid 70s multiplayer EM games seem to have some of the most challenging issues...