I'm having the same problem. If the car is cold. Took it out twice today. The X drives fine. About 20 min. later its slipping in 1st or 2nd gear. The dealer wanted to replace it @ a cost of $6700.00. We leased a 2008 335i today. I keeped the x. Did not trade it in. I couldnt just get rid of it. I feel its not the tranny. And if it is. It can be rebuilt.

What was your outcome on your Xs. I can try adding some fluid. But will that fix the problem.