If I were you I would put some effort into finding another 90-92 300CE with which to swap EZLs (ignition control module) for a few minutes. I say this because they are expensive new and hard to come by used. The symptoms of a bad one can vary to some degree but it is clearly a known problem on your car. Try a search - seems to me there's several good threads on the subject. There are several companies who test and/or rebuild them, e.g.:

http://www.4mercedes.com/ezl.htm