if you change the plugs and you are still experiencing this, try cleaning your mass air sensor. they sell a specific cleaner at parts stores just for this. When i bought my used cai it didnt come with a cone filter, so i used a really old one that fit i had laying around.
about a week of driving with the old dirty one and my car started having some hiccups while cruising and idling. it felt like it does when you have the ac cranked and the compressor has its tick now and then... but my a/c wasnt on!
I cleaned the maf sensor while putting on a new cone filter and reset the computer and its better than ever.