misfiring.....sparkplugs?

i had really bad misfiring before but i figured that was because the plugs i was using were at least 60k old. so i put in a set of denso platinums and the car ran great, no misfiring at all. now im starting to feel/hear the same misfiring, but it has only been 5 or 6k miles since i put in the new plugs. what could be causing this?