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My X3 used to do this on cold start. It was something about how the car is programmed to idle higher on startup for 30-60 seconds, but something was screwy. The revs would jump from 700-1200 and back for 30 seconds, and if I put the car in gear it would surge. I got a software update for the transmission and transfer case which mitigated the issue, but it never fully went away until I unplugged the exhaust valve actuator.
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