did you change the plugs yourself? if not, i hope your mechanic was not so incompetant to let debris fall into the cylinders. is the car suddenly using more oil?
i would take out the k&n filter and go w/ a stock paper one.
if it threw a code previously, it should be stored. might need to take it to a shop w/ dealer type tech tools.
try a good quality fuel system cleaner like amsoil or chevron. does that car have an EGR valve?
check this out. might help you find a reputable shop in your area...
http://cartalk.com/
about half way down it asks you to give your zip code. best of luck!