Hi, this should be an easy fix:

First do the water temp sensor (located on the driver's side, top rear of the cylinder head):

http://catalog.eautopartscatalog.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=1Z40YZDXM1Z40YZSCO&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1991&product=C6020-12315&application=000041722

If that seems to help, but it still doesn't seem quite right, do the OVP:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1Z40YZDXM1Z40YZSCO&year=1991&make=MB&model=300-SEL-006&category=All&part=Overload+Relay

These are the first two parts I changed on my first 300 SEL and it was behaving exactly like yours. It starts like new now.

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(2) 1988 300 SEL W126