If the newer MBs are anything like the older ones, the proper tire pressure should be listed on the inside of the gas filler door.

I'm not about the options on the newer Benzs, but if your model has variable assist power steering...it could be possible that it's malfunctioning and giving you too much assist, as if your doing low speed manuevers like getting into a tight parking space. A system such as that should reduce assist at higher speeds such as highway driving. It's only a thought as I am not sure what's on the 1997 E420s.

Nolan