Check the ground to the starter solenoid. I had hell with this same problem. In the end I had to run a ground from the solenoid ground wire at the terminal in the engine compartment to the engine, which I know is grounded well. I understand that the grounds on my 1975 115/300d all end up under the dash at a common point and that sorting out the proper ground is a 2 or 3 man job and not for the feint of heart.