No further diagnosis is possible without a vacuum gauge. A Mity Vac is the tool most people prefer. You need to T in the Mity Vac to the transmission modulator line, run the hose to the Mity Vac out the cowling and rest the Mity Vac on the windshield where the gauge is visible and drive around recording the vacuum levels as the car accelerates and shifts. Report back the results.

__________________
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