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Too little info to tell but one educated guess is an EGR valve fault, this will certainly manifest itself when going uphill.
The copper injector sealing washers can go brittle & cause problems but usually it'd misfire with this fault & sometimes smell of diesel.
The 2.7 5 cylinder lump is generally regarded as one of Mercedes best diesel engine.
Oh & if you give us an idea where in the country you are its easier for forum members to help or advise one another.