Poor connections somewhere. It's not an unusual problem, you'll find a fair number of posts on the topic if you search here. Some people think it is a result of drops in voltage through old connections and have installed a relay next to the battery and activate the relay through the ignition switch which then sends full voltage to the starter circuit. There's a picture here somewhere of such an installation.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13