Sticky Speedometer

Hello,
I just purchased a 2002 X5 4.4i Executive edition.
The Car is in GREAT condition with only a few minor repairs required.
I didn't notice however (and neither did a bmw 100 point check) that the speedometer has a problem.

It will tell my speed but it does not go up fluidly. My Tach goes up fluidly but not my speedo. What happens it it lags and it will suddely go to 30, 40 etc.. but it doesn't go up fluidly like it should.

Any advice what to do? I wouldn't mind having this working properly but will wait for it to die if the repair is too much money (or I could simply put a GPS unit in).

Thanks!