i have the same thing on my 99 with 126k on the clock. ive just have given up and just let it go. i think mine is a bad plug or a bad injector....both which will be brand new in a few weeks.
other than that i tried the seafoam thing, i think it might be a plug going bad, or i guess it depends on your milage, i think it could be one of my injectors, but the truck runs just fine and it has never thrown any codes.
1999 GMC Sierra SLE ECSB .... 5.3L w/ Slight Performance Tune / True Duals with Flo Pro Glass Packs.
Project ''Bad Company'' Future = 6.0L (Forged Internals) / Boost (TVS) / 4L65E LvL 5 / AWD Conversion / Full LT's with Cats / Methanol Injection / Lowered 2" / SS Rims / Frame Up resto
Officially Employed!!!