Dirty fuel filter? Wouldn't that be more apparent at higher RPM and greater amounts of throttle? It seems to have good upper RPM performance but I have not really pushed it past about 4500RPM. The performance does seem a little soft on the bottom but I was thinking it was caused by the transmission upshifting so fast.

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2004 (05/04 build) X5 4.4i - Silver/Black - Premium Package, Sport PAckage, Rear Climate Package, 19" Wheels, Navigation, Park Distance Control