It was down to a set of faulty wiring looms - some went to factory fit, some went to retrofit. It seems that people who retrofitted noticed the problem. My E46 had this exact issue and it took about 6 visits to the dealer to resolve, and only because I found the info about the wheel sensor wire did it get fixed.

But it may be something totally different in your case, however unlike the early Mk's of navi computer, the Mk4 has no dealer calibrations - it takes all the data from the GPS and the inertial sensors in the car (gyro, speed and direction etc) to calibrate