i have the same problem with my 86 190e-16. and i've taken it to so called mbz repair shops and nobody seems to be able to diagnose the cause.
my mbz has a diagnostic device, which some kind of a tool plugs into it and it will eventually give me a code which in turn tells me the part in my car that is failing.

What i want to know is if you guys carry this tool and for those of you who are technicians will it diagnose my problem?