You know man, I have the same model and now at 82K miles... my car has a little rough idle but nothing as worse as you are saying and it just drives rough.. Mine is a CPO and BMW techs have checked it out many times and they say - this is normal for E53 blah blah and they are loud and rough to drive and idle... So i am at a loss as well. I test drove an E70 and whoa loved that baby, unfortunately still too expensive for my wallet. Good luck finding the root cause

__________________
_____________________________________________
Present Toy:
2008 BMW X5 3.0si
(Sport Package, Technology Package, Space Gray with Black Nevada Leather)
_____________________________________________