The most common cause of a slipping transmission is low fluid. It could also be worn clutches in the transmission or the torque convertor. Does it slip in all gears or just road gears. Is it all rpms or say 3,000 or more?

I would pull the fill plug and see if a small amount of transmission fluid come out. There are two factions regarding changing transmission fluid. Do or don't do it. I change the transmission filter and fluid every 50,000 miles. If the fluid is low I would change it with the proper ZF fluid. However, the major of the times the fluid is changed because of a transmission problem it is too late to be the fix. Is there any unusual noise or vibration when it is slipping?

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas