I recently went thru the same scenario, on a 1993 300E. Would require 2 cranks to get her going when temp. below 40. OVP was replaced, & fuel pressure was good internally, everything else perfect. Donnie from MB Autowerks got a second look at it, overnight, went thru it, and traced it to the battery . I said no way, it is good. Was about 2 years old, tested fine...until under load. Then it would crap out. Dropped a new 850 CCA Interstate in, and all problems went away.

Simple.
Good luck
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W124
Sportline Swaybars
Bilstein HD Struts
16" E420 wheels