What tire pressure are you running?
Are you increasing the pressure according to the diagram on the gas cap cover?
Also, what kind of tires do you have?
I had a '95 E320 wagon with Michelin MXV4's and never noticed the problem you describe. I always upped my tire pressure when traveling with a full load of people and cargo.
Higher tire pressure might help.

------------------
'98 E430
MBCA-Peachtree