egr leakage. strange parts, like aftermarket ign modules, distributor assembled wrong, plugged muffler. in all the years whats been done?. see somebody with a tech one to get started. live datastream during road test can show u things u cant otherwise measure. ive seen sensors and actuators make noise electronically that make pcm/ecm retard timing and change fuel trim. a vacuum gauge can eliminate exhaust restriction as poss cause. a fuel pressure gauge can show readings if taped to the windshield. library has free tsb info. hope u find it quick