Replace the OVP relay and see what happens (behind the splash cover behind the battery). Cost about $70. When it's bad, the voltage gets cut off intermittantly to the fuel mixture control, the ignition, and the cruise control, etc. The relay is there to protect from excessive voltage, but when the contacts go bad or the solder joints go, it goes intermittant. Fuel mixture will go all over the map as the current to the EHA floats all over.
New O2 sensor is probably a good idea too.
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!