If your car has an overvoltage protection relay, and I imagine it does, it could be on its way out. In the LHD version of that car the relay is tucked between the battery and the firewall. Maybe look there first.

The relay looks like this -

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1UF05A51E1XD0QGLZL&year=1991&make=MB&model=300-DT-003&category=All&part=Overload+Relay

Sixto
93 300SD