sounds like carbon buildup. take it to a shop and have the pros do an entire upper intake cleaning. its well worth it. may want to check your vac lines for cracks and buildup as well. pcv valve is only a couple bucks, so it would be wise to get a new one every time you change the oil. funny thing about lums, the intermittant problems will most likely "never" stop. there is always something that needs to be cleaned or replaced. good thing i like working on my car...