Worked a lot on these engines.if you have an engine management light coming on when you have this problem,stop the car and turn off engine,restart and the light is off again sounds like an E.G.R valve sticking,I`m not saying this is the fault but I`ve freed off dozens of these valves,causes engine to smoke & bad performance,a code reader would maybe help you out too.Valve is on back right side of engine looking from the front,may have to remove air intake hoses to gain access and swing fuel filter to one side and remove three torx screws from valve,get some wd40 or some oil down into valve and work back and forward till it is free and the spring closes the valve by itself.Good luck hope this helps you.
Neil
April 2010
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