I had the same problem before. The Mercedes garage in Austin told me that there was a leak around the transmission front pump and it would cost me $900 to re-seal the pump.
Later I found out myself that it's not the front pump. It's the transmission pan gasket which is leaking. It took me around 2 hours to drain the fluid, install the new filter and install a new gasket. The cost is $24 dollars.
After this, no more leaking and the transmission runs like a champ.
I was even thinking to have my tranny rebuilt which will cost me more than $2000.
I used to have the problem of hard shifting. After the new gasket and fluid change, all of these problem went away.
The leak is really hard to trace. Sometimes, it seems to be the front pump but you want to make sure this is really the case before you spend big bucks.
Hope this help.