I just connect the same color wires to each other in the shift coupler access in the tunnel. I never checked to see if it works with them connected via different colors. I've noticed that the magnets in the speedo sensor ring can collect metal filings over time, so it might be worth it to clean off the buildup of metal sludge on the sensor ring. You can rotate one of the wheels with the car in neutral and wipe off the ring thru the sensor hole.
The G50 sender is not the same type as the 915. So don't go looking for a used 915 sender to see if that fixes it!
The both do the same thing, but for whatever reason, Porsche changed the sender on the G50. There's actually a TSB from Porsche on the original sender.