Its your torque converter as I just dealt with this issue myself. See post 24 in this thread for my story to conclude this as the issue.

http://www.xoutpost.com/bmw-sav-foru...-tranny-3.html

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2002 X5 4.6 Sold
Dinan CAI, TB, Mass Air, Stage 4 engine & tranny software, X-pipe
2004 745Li
1997 Ford Mustang Cobra Vert
1985 Mazda RX-7
13B-REW single turbo 60-1, Haltech, Meth injection, MSD Ignition, and much more
2008 Mazda Speed3 Hypertech Tune