I take it that yours is a manual box
http://forums.mercedesclub.org.uk/showthread.php?t=25928
This is the only mod for this
OK dont do this mod on an auto, Auto boxes are designed to hold onto the 1-2 gears to warm the CAT up quickly this is normal
This can happen sometimes, why not change the gearbox fluid and filter, its worth doing as most never get done, everyone says that they get smoother changes after doing it