406 hdi 2.1 reving problem R plate

sometimes on or after start up,the engine revs on its own ,either a high rev or,a alternating rev from idle to high then back again ,over and over,we cure this by switching off then restarting and sometimes we have to do it more than once.the car is running fine most of the time,but this fault can acure anytime on start up or just after.can anybody help