My 2 wheel drive sapphire has a slight misfire I start it from cold and it's like it's flooded on 1 cylinder because after a rev it clears. As you drive along sometimes it seems to misfire on boost like a flatspot It's hard to describe but it then clears and off you go. The car generally runs sweet apart from this misfire/flatspot

I've changed the spark plugs, leads, rotor arm & distributor cap but it's not made a difference.

What might it be?

Thanks