There is more than 1 ailment with your described symptoms and, in lack of a reliable "crystal ball", you have to provide the necessary feel for what your engine is telling you.
Then we can quit guessing and try to interpret the symptoms.
Please try to describe the vehicle, it's recent history, how it acts under what conditions and of course what you have determined in testing it.
Your more recent statement brings to mind the simple problem caused by NOT CLEAN (fouled) spark plugs trying to digest our current available fuel-especially on a intermittant vehicle use where the fuel has a chance to degrade while in storage.Our gasohol changes quickly in storage and will short out plug insulators readily during operation when the engine is not warm and mixture is rich.
