You have the famous D-jetronic system. I'd suspect a vacuum leak or dead spark plug cables before jumping into the injection system, though.
Did you replace the spark plug wires, cap, and rotor? If not, unless they are fairly new (less than three years old), I'd replace them, too.
The gap spec for 1973 is 0.6 mm rather than 0.8 -- you may eliminate the problem by re-gapping the plugs. You don't have a high voltage ignition coil, by the way, and the orignial Bosch plug number (old series) is W175 T 30. Gap specs for 1980 and later will probably be too wide.
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!