The question is, Does the transmission slip or just shift funny? The diesel engine cars have a very complicated vacuum system that controls the modulator to shift the tranny. Not very many people can get these right. Also it will not shift properly when cold, it will stay in a lower gear until warmed up, I had a 280C that didn't want to shift up into 3rd until the engine was warm, but remember to be very gentle with the car when it is cold, don't rev it out trying to warm it up faster, that is not good for a diesel. If it is a shifting problem then it is adjustment required, if it is slipping, it either needs fluid or it is fried, and I have heard sometimes when you change fluid after it has been in there awhile can cause the debris to loosen and mess up the tranny, but that takes awhile, it is unlikely it would just start slipping immediatly after a fluid change, that would take awhile. Report back and let me know it you have shifing or slipping problems. Good luck!
By the way that 300CD is a great car.
Brian

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Brian W. Heitman

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