I am not 100% sure, but it sounds like you have a Sagem dial. My Fidji has one of these and the needle on this shakes erratically until you get about 40mph and then it is still. Its never bothered me as I have GPS to tell me the speed, but I can understand your concern.
You can tell what speed you are doing fine after a while but it takes some getting used to. I am not sure whether they are built like this or whether it is just a fault, that may be common with the Sagem dials.
Both my RT's don't have the Sagem dials so I can't compare it with them, they are totally different.
I have heard from a couple of people that it may be a "wire" speedo or something, and may be meant to be like that, but I dont know enough about speedos to confirm this.
I hope that is of some help