A piece of grit, carbon or maybe a small spot corrosion on the valve seat or the valve mating surfaces shruggyThe valve not seated all the way will do the same thing, check the valve guide to stemm clearances also scope If it is more than .0030 fix it up I've seen the same reults many times in the past shruggy

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)