The 722.5 trans is simular to other Benz transmissions except for Overdrive unit.
There is a control soleniod for 4-5 shifts & its problems have been noted on "RED diagnostic fische" that dealers receive. Sounds like this solenoid is causing your problem. The car has a computer that controls & monitors 4-5 shifts, has the computer been scanned for codes?

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MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS DEALER M.B. TECH
190E 2.3 ITS RACECAR