2010 Mazda 3 "Soft Skip"
 

My wifes 2010 Mazda 3 seems to have a "soft skip" when ideling and hot, seems to puff slightly through the exhaust. When it's cold there is no problem. It has thrown a P0303 code several times. If I pull either the wire harnes to a coil or injector it misses completely on that cylinder. I have changed the plugs and swapped the coils to different cylinders. (1 to 4, 4 to 3, 3 to 2, 2 to 1) with no change. When it throws a code it's still cylinder 3. I have run 3 cans of Seafoam in the tank and it hasn't thrown a code since, but still has a "soft skip" at hot Idle.
Could this be a bad injector or something mechanical like a mis-adjusted valve?