Before going to dealer, try finding an autoparts business that lends a code reader to see if any or what code(s) are thrown.
Here's a link to some N54 codes.
http://www.n54tech.com/forums/showthread.php?t=3930
I'd also recommend new sparkplugs if they are the originals. Most boosted cars change plugs between 20k and 25k miles.
http://www.burgertuning.com/N54_BMW_...t_adapter.html
Good Luck!