Here is a good explanation of how the automatic transmission works:
http://auto.howstuffworks.com/automatic-transmission.htm
There are several pages you'll need to go through to see the whole explanation. Just don't forget to hit the "next" button at the bottom. Pretty neat moving graphics.
There is an outfit in L.A. that overhauls MB xmsn's exclusively. I think their prices are a little better than that mentioned above. Their link is:
http://www.mercedesdismantlers.com/
It sounds like your B2 valve is sticking or close to breaking. I've read of others replacing that valve mechanism without totally removing the xmsn. It might be work looking around.