Yes, flex disks probably the first thing to check, front and rear on the propeller shaft. They could be dry/cracked/separating, as mine were. Also, not sure on your motor (m112?) but the m111 in my 230 has a belt tensioner and shock that can vibrate with age/wear, and in my car, it caused the EXACT same symptom you describe under the same conditions.

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former: 83 300D, 97 C230, 93 400E

current: 08 C300 Luxury , 92 500SL