Set the wipers at the mid point of the wipe by turning off the ignition whilst the blades are moving, you may have to do this a couple of times to get it to stop where you want it. Then you remove the blades, being careful not to drop the stalk against the screen with a bang. Take a a couple of small pieces of leather to protect the metal and using two pairs of pliers twist the end of the stem so that it sits flat on the screen. You will note that as standard only one edge is touching the screen.
Because of the angle of mounting on the Morgan the angle of the end of the blade changes over the length of the sweep. Setting it at the mid point is the best compromise. In this way the blade is able to flip from side as it changes direction. The vibration and skipping occurs because the blade has not flipped for the return stroke.