X6M please help

[57vanish]

hi there
i am new here , seeking your help
i have got a 2year old x6m which has 20000mile on it
about 6 month ago the car had a strange problem with it
every time shifts form 2 to 3 a jerk feeling through the car
it feels the rear brakes are being applied on/off quickly. its a really subtle loss in power which lasts for a second.
for the problem i had the dear upgread the software ect. but with no help
is there any one had the same problem ? what should i do ?

please please help
and sorry for my bad english