jc2

I've got a 04 Cavalier 2dr 5sp manual that has a speedometer that bounces slightly when driving down the roadbut seems to register the speed pretty correctly. Today when I first took off it would only register between 10-15 mph but after shutting the car off and restarting it went back to its bouncing thing but seemed to register pretty close to the correct speed. Is a there a plastic gear that might be going bad and can be replaced? The vehicle has about 41000 miles on it and runs out very good otherwise.