I'm with Mr. Brotherton on this one. The wiring harness is a well known issue on those cars, and can cause a litany of problems. Replace it. Talk nicely to a dealer and see if they'll cover a portion of the cost.
They'll give you some BS about your car being the first one they've EVER seen, and refuse, or maybe not. Worth a try.
Next, check the OVP and see if it has to go. Again, this is a well known trouble spot, and is easy and quite inexpensive to remedy.
Once those two things are out of the way, revisit the diagnostics. If the car is still running poorly, then diagnose the problem and don't just toss parts at it.
Both the wiring harness and the OVP cause the symptoms you have, and since the harness is shot, go ahead, or risk further trouble.
1998 C230 "Black Betty"