I'd say to start with that fuel filter. Low hanging fruit, easy to pluck.

When was your last valve adjustment?

Any fuel leaks?

Also plan to check your glow plugs. That was what my mind went to when you described the symptoms - a failing glow plug.

__________________
1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

http://i.imgur.com/LslW733.jpg

The Baja Arizona Oil Burners Send a message if you'd like to join the fun
Left to Right - UberWasser, Iridium, Stuttgart-->Seattle,, mannys9130

Visit the W123 page on iFixit for over 70 helpful DIY guides!