Turn the heater off by pushing the far right button on the control. If the condition improves, you have a vacuum leak in the heating system somewhere. If it doesn't look elsewhere. Could be flex disks but you need to exclude vacuum leaks. Ideally, you can tap in a Mity Vac and monitor the vacuum to the transmission as you drive.

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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