I have the same problems sometimes in more gears then just 6th. I was told the new transmission software would fix it so I am going to get that done soon and see of that is the solution. I have seen that some other members mention the same problem, but I really havnt seen a solution yet short of rebuilding the trans. I'm hoping that is not the case.

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01' e53 X5 3.0i - 140k miles
06' e90 m-tech 330i - RIP
09' e82 135i 11.3 1/4 mile @ 123mph - RIP
06' e53 X5 4.8is Le Mans blue - 400hp/410tq