With the basics covered, it might be your fuel pressure regulator is leaking. It is inside the manifold, you have to take off the upper manifold to change it. There is also the EGR valve which might be causing it. You can remove it and clean it out with carb cleaner or such and if the problem is because it's sticking, that could fix it. Or go to a junkyard and get one hopefully cheap. New ones are ridiculou$.

 

How about the wrong spark plugs? The iridium plugs called for in the owner's manual and local parts houses are .30 longer than the platinum AC Delco 14 that came out. Didn't catch that until I pulled a plug to see how they looked. The end 1/4" of the threads showed debris! I'll be putting the platinum plugs in tomorrow.
Jeff