I would make sure the plugs you are using are exactly what is specified in the manual. I was getting idle issues with my 95 E420 and learned that a prior owner had installed resistor plugs instead of the Bosch F8DC4 plugs that were specified. Installing the correct plugs solved the problem.

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