I have a 2002 Ford Focus Zts Zetec and it first started bucking when I went to come to a stop but was fine while accelerating, I then fixed the ignition coil, spark plugs,valve cover and gasket, cleaned the Mass Air Flow sensor and spark wires. When I had it tested, it said that there was a misfire in the fourth cylinder and I have no idea what else could be wrong with it. After all that has been fixed there is still a really rough idle... I'm close to giving up. Help?