Try swapping the fuel injector to another cylinder and see if you get a different code. P0303 indicates a misfire on cylinder #3, the last number of the code tells which cylinder. My wife's sebring went thru this in January with a misfire on #5. After many things replaced I had the garage swap the injector to another cylinder and the code went from P0305 to P0306 because they moved the "suspect" injector to #6.
I know its some work to swap injectors around (plenum removeal, etc...), but to me it seems to be the next logical step, since beyond that, more serious problems would be suspected.