My dad said the he thinks its the flex plate, it cracks from the inside out youll have to pull the transmission and converter to do it.. 2-3 hrs to do but it will tell you for sure.. Since it didnt change with plugs out he knows for sure its mechanical on the outside. What happens is the crankshaft starts dancing inside when its got some miles on it and the converter wont move so the flex plate does exactly that, flexs. Id say its probably that..