Well Geoff, I don't know if you've ever driven many other European cars or at least non-American cars, but I am yet to find many cars that ride better than my 190E.
Some I've driven recently I can remember are worse riding are E46 BMW 318i, '00 Volvo V70, '98 Honda Accord, any VW, any Audi except A8 and possibly new A6. I don't think any Japanese cars do except some Lexus.

Large BMWs tend to ride better, and newer Mercedes including the 300E.

I can't remember the ride on the C-class, but it hopefully isn't much worse than a 190E - you could never describe a 190 or 300 as firm.

So perhaps you have a problem? Try and ride in some other MBs to get an idea.

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190E's:
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black 2nd owner
http://www.maylane.demon.co.uk/190esmall.jpghttp://www.maylane.demon.co.uk/190esmall2.jpg