You might want to double check with the mechanic, but it sounds like he may have just firmed the shifts up via a turn or two of the modulator. This is only a bandaid. As mentioned, friction materials eventually wear out. The cylinders can also start to leak, causing them to weaken in their function. Also, the valve body can get varnished and plugged by old coagulated sludge; you would never see this without disassembly. I speak from experience, having had to disassemble and clean mine out on the 300E a year and a half ago. Those tiny check balls can eventually bind.

Matters are worsened when the trans fluid is not changed religiously every 30K. Excluding maintenance history, given the mileage on your tranny I'd say it's quite possible you just need to break down and get a rebuild done.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280