megane ii cuts out when cold slight missfire aswell

hi I have a megane ii 1.6 changed the dephaser month or so ago .This problem was there befror the change .when I start the car from cold it can take 3 or 4 attempts to start here without accelerating and I get a misfire for a min or two .when I start here it revs up to just under 2000rpm then dies what can be causing this to happen ..thanks