You've sure covered all the usual suspects.
Checked the ignition system? Plug wires, ignition components?
Mass Airflow Sensor? Does it read a code on pre OBD-II cars? I don't know. Our Mazda did the EXACT same thing, and it was the MAS. Before you get shocked by the MB MAS price, I'll tell you that a new Maxda one for our 1988 626 was $1300. The dealer GAVE us a used one for nothing...
I'm at a loss. With new harness, OVP, and O2 sensors, this is a bit of a stumper...
1998 C230 "Black Betty"