Misfire on a v8

hey up my friends,got myself a intermittent misfire on my v8 3.9 landy, I have taken all the silencers off obviously to make it sound good,got the 4-2-1 headers then a 2 foot piece of exhaust and it sounds amazing but noticed sometimes it only seems to fire on 4 cylinders, so do I need to put 1 silencer back to create back pressure to stop it misfiring,any ideas thanks