Larry, my friend has your same car. '81 300d.
Same problem but not symptomatic with cols weather, just did not start
but glo plugs o.k. Did the fuel filter basics etc. no problemo.
Ended up changing the fuel
pump for the heck of it cuz he got discount at dealer parts.
Bingo. Unexplainable to him because
he saw that fuel came out of the line o.k. prior
to replacement. Says it has better pick up now also.