Have you checked the fluid? I'd also check to see if the bowden cable is sticking. Seems like you could cover the 300 miles on secondary roads at lower speeds in 3rd gear as long as it stayed in 3rd. Think of it as driving a 240d.

__________________
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