308 gt 16 plate feels missfire?

1sr 2nd gear fine, but top end of 3rd 4th 5th and 6th feels as though a cylinder isn't fireing well,still picks up speed but judders,put new plugs and air filter in with no luck, car still pulls really well whilst this is happening tho,ive been told could be one of the single coil packs on one of the plugs