mine decreased noticably after the engine broke in...still notice it occasionally. As far as the dealer is concerened, I got the standard "couldnt duplicate problem" rubber stamp.

What I meant by programming is it seems to me that the transmission downshifts at the wrong time causing the hesitation and restance we feel. I think this can be adjusted by reprograming the transmission. A software update can fix this. Persistance in asking the dealer will evidentially pay off.
I had other transmission issues earlier that were fixed w/ an update (mine is 11/06 build)

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2013 X5 35i M Sport