Sounds like the E53 tranny "lurch" issue. Search tranny lurch in the E53 section. Tons of info on it there.

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2005 BMW X5 4.4 Sport Upgrades:

Dinan Cold Air Intake
Dinan Stage II Software
Akebono Euro Cermaic Pads
Brembo Rotors
Titan Silver Metal Gauge Rings
DICE IPOD Module
Hardwired Passport 9500ix Radar/Laser Detector