It sounds as if you are going lean on throttle actuation.
Check for vacuum leaks before jumping into the EHA. Also, you need to check the EHA current, it must correspond to the duty cycle or you may have a wiring or computer problem.
Certainly check the vac connection to the computer, if you have one -- bad info here will do funny things to the mixture.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!