Common problem, and why you can never really trust the mileage on 124's. When mine did this I JB Welded a metal flange across the two ridges on either side of the little gear. This keeps pressure on the gear to keep the pin in the hole. Works fine.

Disconnect the battery before you pull the binnacle. I shorted something out on the circuit board that made the oil pressure gauge dead and the tach goofy. I had to locate another board and swap them out. PITA.