I believe it is possible that the 'amount of air from the fan' problem could be vacuum related. Does the fan seem to blow as hard but not as much air comes out? On my 85 300td, when the fan comes on, a vacuum actuator is opened immediately behind the glovebox. I suspect that if this actuator does not open or does not open completely, it could modify the air flow. This may account for why it blew hard when the system was changing from one mode to another since at that point vacuum is rerouted.
It should be fairly easy to pull a vacuum on the actuator behind the glovebox while the system is operating and see if it makes a difference.
All this assumes that your system is somewhat similar to mine.

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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