Could easily be a bad connection. Mine hesitates for a second before doing anything when the engine is cold. After the first start of the morning, it returns to normal. I believe it is a bad internal connection. Last week at 11 below zero, it would not do anything at all. Later in the day when it warmed up, it worked after holding the key in the start position for 2 seconds.
I've got a new (rebuilt?)starter on order from Advance Auto--$100 lifetime warranty. No one else I could find offered a lifetime warranty.

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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