Olivier,
I had what I think is a similar problem on my 98 210. I had very poor performance and rough running behavior. I got many codes most of which were EGR and MAF related. When running worst I also got a "multiple cylinder misfire" code. After replacing both (MAF,EGR) more than once I finally took it to an Indy who had a scanner that would pull more codes than my Ebay version. He came up with a "driver error" code in the engine control module. Turns out one of the drivers for the engine management computer had gone bad and was creating all kinds of strange trouble codes to be tripped, and causing the horrible performance. I got the control module reprogrammed and it has been great ever since.

I thought this might be something to look into. Good luck, when running right they are great cars.

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Muleears
'07 E320 Bluetec 133K my DD
'04 Jaguar XJ8 VDP, 34K
'10 Hyundai Accent 60K Grocery Getter
'02 VW Golf soon to be on the road again
'97 E300 Diesel Son's DD
'61 VERY tolerant wife

Hampton Roads, VA USA

Gone but not forgotten:
'67 250S 95K
'86 300SDL
'87 300D Turbo, 364K! R.I.P.
'98 E300 Turbodiesel, 213K
'02 S420, 164K
'01 Prius 138K