Rev counter

i have a renualt 02 laguna,the problem is that when i start the car the rev counter shoots up to between 2000 and 3000 revs . the xcelerator pedal has no control over this,and the only way that i can stop the engine revving is to switch the car off, and switch it back on . this may take several attempts before it returns to normal. it can also happen when i am driving , and i have to stop the car and repeat the same as above again.