firstly i would do a full service on it , all filter and fluids , oil/coolant water,fuel filter(s) and air filters
pay attention to any water seperators , check all fuel line connectors and unions are tight and in good order ( drawing a small amount of air into the fuel system if often the cause of ''a lack of power '')
check your diesel as it absorbs moisture and can severley effect the running of an engine .
if this has no effect
get a rag /goggles and the right size open ended spanner and slacken each fuel injector nut whilst the engine is running , listen for the engine noise going down , if it dosent this means the injector is deffective ( might need replacing or cleaning ) do this to each injector in turn , be very careful as you do this , the diesel is at 200bar pressure and can injected you ( hence the rag )
a final one is the fuel lift pump may be deffective , they often partially work and starve the engine at maximum revs , depending on your engine this will be an electric one or driven directly from the engine block , just follow the fuel lines from your diesel tank ,.
hope this helps
on Jun 04, 2013