what is wrong with my t reg peugeot 406?

my 406 over revs when i stop at a junction , when i first start it on a morning it is fine it ticks over as it should but after a while whenever it is in nutral or i press the clutch in the engine steadily builds up the revs and idles at 2,000 plus ,sometimes it goes up to over 4,000 ahd i have to turn off the engine at traffic lights ect . what is the most likely cause cheers