Typically this problem is caused by a vacuum solenoid that is integrated into the top of the injection pump. It actually actuates the previously mentioned stop lever. It is very easy to replace. I just did mine a couple of weeks ago, and it took all of about 20 minutes to do.
\'87 Mer-B 300D (220,000 miles) (sold)
\'93 Mer-B 300D (173,000 miles) (sold)
\'87 Porsche 928 (101,000 miles)
\'98 Dodge Ram quad-cab (250,000 miles)
\'2001 CLK430 Cabrio (50,000 miles)
Past cars: 1969 mercedes 250, 1981 300D, 1972 220D