Hi - research the Bowden cable adjustment, modulator valve, kickdown connector. There a manual here: http://www.w124performance.com/docs/mb/other/tranny_722_repair.pdf - see section 27.51 Section 1.3/1 control pressure adjustment and related shift and throttle pressure controls. The unprompted downshift suggests that the throttle pressure and/or vacuum control is not regulated properly. The clutch that is affected is the KW, but as I understand, that is a robust unit and its more likely to be an issue woth the adjustments when the trans was installed. The linked file has shift point pressures of you di find a reputable service tech who can test the pressures using gauges while operating the vehicle. See Sec 3.2/5 http://www.w124performance.com/docs/mb/other/tranny_722_repair.pdf.
Good luck