Speedometer lags and shows 5-40 km over
 

Hi
I recently bught a a4 8E B6 2001 mod, and suddently the speedometer started lagging and showing anywere from 5 to 40 Kmt/h faster than im driving.
When i park the car the speedometer stays at 5-40
If i leave the car for a few days it slowly drops. :confused:

Does anybody know what might be wrong and how to fix it?

Have been googeling for hours without finding any answers :(