There were issues with some 6 speed transmissions built prior to Dec 2003 popping out of first gear. The early fix was to replace the entire transmission. DC has since issued a TSB addressing the issue with a first gear syncro kit fix. Many drivers still complain the six speed is not very smooth and is notchy as have several automotive reviewers. Mercedes Benz has never been known for a smooth shifting transmission and I agree it is not the best it could be. IMO the clutch takes some getting used to before smooth shifts are possible.

You can also do a search on 1st gear popping out of gear within this forum and come up with a lot of information.