2001 - C200 CDI - intermitent loss of power

Hi all,
I am experiencing looses of power on my 2001 C200 TDI. Sometimes evrything is fine, sometimes I have the feeling that my car has only 40HP !!
impossible to drive faster than 60mph on flat highways... and impossible to to reach 40mph on climbing roads !! the mechanics have done tests by plugging their electronics dignostic casetool on the engine but did not noticed any anomalies.
Any clue ? any tricks ?
thx br

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