Speedometer not accurate on my 2001 Renault Megane 1.9dci?

Hello. I'm having an irritating problem with my speedo. It displays a speed which is not accurate, sometimes too slow, sometimes too fast - often it will start out OK, then during the journey becomes more and more inaccurate.

Also, what often happens, is that while travelling the needle will "tick" and that speed then becomes zero. this can be reset when stationary by turning the ignition key from position 0 to 1, or from 1 to 2 as many times as required to "reset" the zero.

At the moment I'm judging my speed by my rev counter, but it is becoming increasingly annoying.

Thanks for any help