What I would do is ask your dealer to write on paper (and signed) about the problem, but that they can't fix it / can't find the cause / already 'fixed'. Then keep the car, and if the problem gets worse you go back to the dealer... they then can't say they didn't know about it (as you've got it on paper).
We have had a problem with our 1995 740i, where after a few thousand kms the motor started consuming oil, every 10,000km we had to add 1,5 to 2 liters of oil! We complained to BMW about the issue, and they said nothing is wrong, that it is normal for such a car :S !!! After 80,000km the motor blew up (from the 8 cylinders, 6 were totally destroyed, 1 was hanging there and the last one seemed ok)!! BMW could not doing anything about it because we did not mention to them about the problem!!!!!!!!!!!!! It was a known problem at BMW that the gasoline was eating the side-walls of the cylinders... most engines from other BMW owners were replaced through guarantee, but we had to pay for it ourselves!
So, long story short: get them to write it on paper!!!