You never drove it before you bought it? I agree with you that since you bought it from a BMW dealership you would expect it to be mechanically sound. Did you know that is was a AS IS sell or was it a CPO? What does the Carfax say? How many owners? Since it sounds like you do not have any type of extended warranty I would suggest taking to a Indy and have them diagnosis the problem. The 4.8 with a Pano-roof is a beautiful thing, I hope you can get this resolved and enjoy the X. What did you pay for your X and how many miles?

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2010 X5M /// R63 AMG...Wifey's 540HP Family Hualer

2001 X5 3.0i Sport 335,448 miles & counting...
SMG M3 Steering wheel
E46 Stainless Racing Headers
4.6is Exhaust
Turner Pulleys
Afe Intake
Brembo drilled rotors
PowerFlex Bushings
Hualingan
30mm H&R Spacers rear
20mm H&R Spacers front
Bavarian Autosport High-Performance Coils
Active Autowerke's Performance Software