My experience is with W201, so we need someone to chime in with the wiring diagrams for your model to tell us which pins to test and where the relay is. Likely, it is the largest box, other than the computer, near the OVP.
Empirically, you could run the car in idle and tap on the relay and see if the car stumbles. Actually, this is how I confirmed a bad OVP on my 2.6. Many simply remove their relay, open the case, and inspect and resoldered the connections. Usually, it is a hairline crack in a trace or metal corrosion on the ground lugs of the relays:
Here is my guess at the approximate order to test/repace. It could use fine-tuning from other forum members:
1) OVP ($$)
2) fuel pump relay ($$)
3) run tankful of strong injector cleaner ($)
4) check valve(s)and fuel pump(s)($,$$)
5) fuel filter and accumulator ($,$$)
6) ElectroHydrualic Actuactor (if appl.) and its gasket ($$,$)
7) injectors and related rubber($$-$$$)
8) fuel pressure regulator ($$)
9) fuel distributor ($$$)
10) and then the list gets really long ($!)
Edit: If the problem is related to ignition components, you can wire an inductive sensor (I simply clipped one off a timing light) to an oscilliscope and clip the sensor around each spark plug wire. The oscilloscope image cross-references with diagrams in the MB repair manual that tell you what could be wrong. I used this to find 3 bad spark plug wire ends.
My experience is based on painful times with a 190E 2.3 and not much better time with my 2.6. I wish you much less pain, Andrew Seidel.