Have you examined all the tubing from the mass airflow sensor up to the throttle body? Don't forget the hoses to the idle air control valve under the manifold.
Any of those plastic or rubber parts that have not been replaced in 8+ years could be brittle and cracking, letting in un-metered air. I have replaced a lot of plastic and rubber on my two '97s, and I am starting to replace it on my two '04s.