Are you losing aceleration?
What does the dealer say?
Have the tech go with you on a test ride.
They should be able to figure it out. That's their job. If they can't, find another mechanic.
2004 E60 530is, Delivered 12/12/03, TiSilver/LGrey, PP, SP,PS, PDC, ACC, FRS, Step, Xenons, clear bra, body colored rear inserts. (Retired, 1983 henna red 320is)
2004 Corvette Z06 Commemorative Edition