Your problem is most likely the pressure solenoid's in the tranny. I believe there are 3 of them and it's a fairly simple procedure to replace them. I think they run around a $100-$200 each. The solenoid's are made of abs plastic and they tend to shrink and get brittle from the heat of the transmission fluid over time. These regulate the pressure inside the tranny and allow it to engage and disengage based on fluid pressure inside the tranny. Here's a link to a pretty good post regarding this issue: http://www.xoutpost.com/bmw-sav-foru...nsmission.html

I had the same problem and the tcc seemed to fix it.