Man I battled that last year and finally removed the heads after trying all the common stuff. Machine shop found that one of the exhaust ports near the valve was rusted through. These were “088” heads. Guy said that is very common. Mine was a ‘97 no salt motor. Good luck.

I tried everything else, intake manifold and gaskets, replaced the CMI’s and everything already posted here or will be posted. Pulled heads and set them on the bench looking for possible bad valve guide or seat. Poured lacquer thinner in each ports and sure enough one would drain down. That when I asked machine shop to pressure test the head. He called me with his findings and bought new head from him. Had him set it up to match the others. Problem solved.