Andy how many miles does your car have? It is very possible that the bushings in your suspension are worn causing the instability.

Toe-In will definitely affect the stability of the car but negative camber, as already mentioned, will only add to the stability. Some Toe-In as necessary but I'm not sure how much and what the factory specs are on the E420.

I would do the following before you start changing out parts.
1. Have the rear suspension inspected for worn bushings
2. Have an alignment done on the car

A good MB Tech will know if something doesn't look right in the suspension.

__________________
Afshin

Current:
02 C32 AMG

Previous:
92 500E
84 190E 2.3 5 Spd