Check the throttle assembly, most likely it is the throttle assembly malfunction. It is very easy to replace a new throttle assembly. I had the same problem twice before, the dealer checked it and replace teh throttle assembly two time for me, everything backed to normal.

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--2004 E53 X5 4.8is - ECE version; right hand drive; Located in Hong Kong; ext. black sapphire metallic; int. leather black; adaptive dynamic bi-xenon headlights; 16:9 onboard monitor w/o navigation; 20" genuine BMW Y-spoke 214 wheels with Bridgestone Dueler H/P Sport tires; Int. rear view mirror w/ compass.