Most likely, the vacuum is not bleeding off the shut off diaphragm with the switch. Next time it happens, pull the brown vacuum line off the shut off diaphragm on the back of the injection pump down between the injection pump and the oil filter housing and see if it starts.

__________________
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