Just discovered that the return spring on the throttle valve (W124) had broken/disappeared. Motion is transferred from the accelerator cable to the throttle lever by a roller that resides inside a roughly kidney shaped slot and it's wider than the little roller that transfers the motion. Without the return spring the little roller is free to float backwards and forwards in the slot, allowing the engine to hunt. Same phenomenon could occur if the ball joints and support shafts of the linkage overall are sticky. Just another possibility.