The key switch in my 84 300D would jam. I took it apart and found the problem wasn't the tumbler, but rather a spring-ball detent at the rear. The spring holder casting had broken and the parts were jamming. I expect this occurs on many cars, based on the poor design. Jump on this soon before you can't turn the key to get it apart. I now don't have a spring detent, so the key can rotate almost too easily. My son drives that car and hasn't said the key ever turned on its own to shut off the engine.

You can find a fuller post by me on another website (google). Interestingly, Car Talk had this same problem asked a few weeks after my fix and they assumed it was the tumbler. Wish I could have called in to correct them.