Fabia speedometer needle flicking

Hi all , i have a skoda fabia 1.4 mpi , year 2001 , the speedo needle works properly when the engine has just been started , when the engine gets hot the needle starts to flick up and down , then finally falls to zero. what can be the cause , where does the speed signal or pulse come from , is there a sensor that supplies the clock with a signal , where on the engine can i find the sensor if any ?

thanks in advance