300ce fuel problem

Hi there,
I have problems with my 1989 300ce. Dark smoke on acceleration, stattering, loss of power, and bad MPG.
This is exactly the same problem which i faced 6 months ago, and last time i changed the fuel distributor.
Now, the same problems coming back, and my mechanics suggesting to change the actuator (EHA?).
I changed the actuator, but problems still persist.
Oh yea, everytime my mechanics adjusting the fuel mixture, the car would run very smoothly. However, after going uphills, the problems would come back.
I really wonder what really is the problem here.
Very desperate at the moment, and really really appreciate any advice or thoughts.

Tks and regards,

BennZ