I have a 05 X5 4.4 and had the same problem and it happened a few time while going uphill after a stop. Took it to the dealer and told them the trnsmission was "free wheeling" and would slam into gear as I accelerated. They ended up reprograming the CPU and it hasn't occured since.
I recommend telling your current dealer to F-off and go to another dealer. We pay too much money to not have them help.