Hi, I have a late 2008 Cooper D that has had exactly the same issues as you, the hesitation, then stalling etc I asked my dealer in the UK, and they couldn't find anything. It was as though a breather pipe was letting in air or the turbo wasn 't spooling up quickly enough. Last month it was noticeable but it had just had a service so I left it.
Went to the S of France 2 weeks ago, and it went it a big way, as in loads of smoke (turbo oil seal?) unable to run under 3k rpm, little power, no boost and 3 cylinders(?) then curtains, all in the space of 15kms. The car is 25 months old and has done 46000mls. It is still in France as I write, the Euro breakdown assistance is another story in itself!!
Will let you know what the outcome is as I'm sure it is directly related to the hesitation, which started about 10-12 months ago.