Mike, as always, is correct about having to replace the entire actuator on the 123 cars. The good news, though, is that they aren't very expensive.
Follow Mike's suggestion and check them before replacing them. I guess I've had better luck, my '84 123 car with over 500,000 miles needed the gas flap one replaced. It's the only one that's ever been replaced and the system is still tight.
Mike is just spoiled with his swanky, big bodied car, they were built differently in many ways. The 123 was built for much less money, so some of the pieces on them are made like a dollar watch. His 4.5 is the opposite, it's constructed like a piece of manufacturing machinery, not the weight, just the methods of construction.