I too have the same trouble with my 94 e320 sedan. I have an extremely knowledgeable tech that went over the car before I purchased it.
Apparently this is a common problem in this car. The cause is usually associated with the EGR valve, wiring harnes, and or computer.
The EGR valve in my car has been replaced and I still have problems with the light. The wiring harness and computer are both big expenses. I've been told that I should not worry about the light.
Unfortunately, not worrying about the light is not something I can do. If you find a fix, please let me know.