Hi

I got a renault 1200 cc 4cyl 8valve from 98 with a defective speed dail

So now and again it goes down and doesn't come up
And this happends when ever it wants

so what can i do to rule out a few things like de speedo itself or the sensor or even maybe the cable that runs betweens these two
I have heard that there is a huge sprocket on the flywheel that runs the sensor is made from plastic ( hard to think that ) and can brake after a few years of getting warm and cold and get brittle
some say its the sensor but this rease a bigger question ,where can i find the sensor

please help me cuase i dont want to get a speed fine anytime soon here in holland

thanks for youre time