It was a balance issue. If you want to duplicate it, take your pressure plate, pressure plate bolts and flywheel to a good machine shop. Have the flywheel resurfaced and then balanced. It may already be balanced just fine, but then the machine shop will balance the pressure plate to the now balanced flywheel and mark both for the installation. Yes its a couple of extra dollars to do but you get to confirm that the flywheel is balanced at least to factory tolerances, that the rotating flywheel pressure plate assembly is balanced so nice smooth motor.
