I have a W202, C200 M111.945 1998 Elegance that has a flat spot on acceleration. This happens in every gear, as well as from pull away.
The flat spot happens just as your foot is off the clutch and you are pressing on the accelerator pedal.
There are no codes, and the following parts have been replaced:
Fuel Pump
Fuel filter
Fuel pressure regulator
Camshaft position sensor
MAF Sensor
Coils and wires
plugs and air filters
Engine Coolant Temp sensor
O2 sensor
I have removed the Throttle body, opened and checked the operation, no wiring issues and it tests good.
EVAP Tests good
SA Models had no EGR valve

After the flat spot acceleration is good, but a bit hesitant, almost like a lean mixture, but O2 readings are good

Any ideas?

 

Definitely try the throttle position sensor. It was bad on my CLK430, causing lagging acceleration. Start the car and open the hood and operate the throttle position sensor with your hand, it should be easy to see that there is a major lag from operation to response when in park.

 

I had bad seals on my fuel injectors, so I replaced them. Fixed the problem. Did you get yours fixed?