A member on a different forum I subscribe to had a similar problem. It turned out to be the crank shaft position sensor. In your case I would check the plugs and wires first.Ive just replaced the engine in my 4.6 97 rangey and haev a low rev misfire. after about 2.5k its gone but before then its abit spluttery and has a lack of power. any ideas people this vechile is doing my head in.