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Another possibility is that the fuel filter is blocked so resulting in low fuel pressure and again poor combustion - replace the filter in any case as diesels are very sensitive to this
also check how much oil is going through the breather by popping off the intake hose in front of the turbo and checking for oil deposits. Diesels will run off engine oil so this could also be making it rich
If its been overfilled with oil it will also produce surging as the oil finds its way back into the engine