I recently purchased a 1980 300SD with 250000 miles on it and I notice that 1-2 shift is hard and 2-3 seems to be slow, the engine revs up, and acts like it slips occasionally and the 3-4 is hard. When I go into R, it goes fast, but D takes a few seconds. Also, at cruising speeds it seem to pulsate. I can't really tell if the engine is pulsating or if the transmission is slipping. I did change all of the engine filters and transmission filters along with the fluid. Transmission fluid didn't smell burnt, but was really dark.
Any thoughts?

http://www.peachparts.com/shopforum/showthread.php?t=158216

Also, search on Superior shift kit.

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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