A lot of references on the web talk of harmonic vibrations caused by a faulty plug or plug wires. So I pulled all the plugs and checked them. They all look good with a light tan colour and they all fell between the recommended 4-10 ohm resistance. The wires also passed muster between 3 and a bit over 7 ohms for the longer wires. A bad condenser can cause a harmonic vibration, but this ignition doesn’t utilize a condenser only an ICM which itself supposedly can’t cause vibration problems.
One more thing to check is the torque converter. I detached it from the flywheel and held it back for this test. Still no change so I assume the torque converter is OK.
Fast running out of ideas. If anyone has a comment send it along to my email address – grhm53@gmail.com
Meanwhile I’ll just have to drive the car and maybe something will break and that will reveal the problem!