2002 Renault Megane knocking noise when starting the engine

i am getting a knocking noise when starting the engine up on my megane. in then stops after a few seconds. the car stalls then shortley after. and dose so a few times. the is a slight knocking noise on tick over only. and miss firs badley at 1500 to 2000 revs. where they drop to about a 1000 the go up then down again. i changed the plugs but still the same. hope you can help thanks adam