Sorry, no experience changing it on D4 but seems a bit suspicious that it works when you start moving. I would think it's not enough vacuum in idle and when you start moving and rpm increase you have more vacuum and it works, which would mean weak pump or vacuum leak somewhere, of course could be a leak inside servo too. Should do a vacuum leak test on whole system and individual parts first before committing to replace servo.
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Currently 8less
2011 Q7 S Line 3.0TDI, 2016 Tesla Model S 90D
8 history:
2006 A8 Sport 4.2TDI quattro SOLD,
1997 S8, reached end of life with gearbox failure
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