You mentioned the viscuous fan clutch. Have you looked at the modification on Jim Forgione's web page? http://pages.prodigy.net/jforgione/MB_vfc.html

This may be a long shot, but if the clutch is not locking soon enough, maybe modifying it to be locked full-time would be an alternative to getting a new one.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles