I drove a '98 e320 wagon that behaved similarly to what you described. It turned out to be due to a bent wheel... If you can do a swap for wheels/tires it would be one quick and decisive test. Perfect for a dealer if you don't mind the expense....
Good luck and kindly keep us posted!
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...Tracy
'00 ML320 "Casper"
'92 400E "Stella"
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