Do a search for problems related to the 124.128 and you'll find way more than you can read on the engine control system. One thing I can tell you is build a code reader/clearer. The parts only cost about $20 from Radio Shack, and it's easy to build if you know how to solder. Plans/schematic can be found here...
http://pages.prodigy.net/jforgione/MB_X11.html (scroll to the bottom).