I have the same issue in my 82-300SD. My engine idle's aroung 500-550. I have adjusted the idle adjust screw. I broke it. A friend and I repaired it by fixing a L flange to the back of the full-load stop screw with the idle screw in the exact same position it was before. We couldnt adjust the screw any futher down. It sputters and shakes a lot until i hit the pedal and then it will settle down back at 500-550 and be fine.