I'm on the same line with EODguy. Check fault codes from GM if there is code for servotronic solenoid. When the solenoid wears it's resistance drops below 7,5 ohm which is the threshold when the code activates and drops servotronic inactive.

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E39 530dA -02 M-Sport Messing metallic
E53 X5 3.0dA -06 Sport Stratus grey
E70 X5 40d -12 M-Sport Space grey