I recently bought a 1976 450SEL with over 200k miles, although the true mileage is unknown because the odometer stopped working a while ago. The car needs to be warmed up for at least 15 minutes before the car goes into reverse. After that, it works fine. If the car is turned off after several hours, then it must be warmed up again. The car shifts smoothly in all gears after it's been warmed up and seems to be normal besides that. Is this a major transmission issue or a quick fix?