It sounds more like a bad OVP to me. As if solder defects had developed so that it drives the EHA intermittently -- leaving you in limp-home mode sometimes. Even in limp-home mode, the car should run well, so I would also suspect other problems in the fuel system such as bad mixture adjustment, vacuum leaks, injector leaks, etc. You can diagnose these things with the manuals on CD, a multimeter, and a vacuum tester.
However, my experience with this problem has been that it will save you money in the end to take it to an experienced MB tech that can isolate the exact problem, rather than throwing parts at it.
Hope this helps! Cheers,
85 500SEC Lorinser
83 300D Turbo