citroen berlingo speedometer faulty?

My Citroen Berlingo speedometer does not work until I reach 30mph normally. If I rev the car in a low gear above 3000 revs it seems to work below 30 mph some of the time. It has had a wobble in the speed shown at about 70mph since we bought it 3 years ago. The rev counter is rock steady but of course has a different drive point.
What is at fault here, the speedo or its sender in the gearbox or hwerever it is?

ian tew
April 2007