my focus does that every now and again as well. on idle will all of a sudden get the ignition a bit wrong if you like then the exhaust will sound like its running mega lumpy cams. this is very infrequent but still does it. however, when I serviced it in Feb, I replaced spark plugs, got Ford Motorsport HT leads and also a second hand working coilpack to fit as I thought my coil pack was going to pack up. Ever since then it hasn't done it at all apart from once the other day