May be a collection of carbon etc in the engine - its worth checking your EGR
valve and inlet pipe to see what condition they are in. My gf just bought a Clio 1.5dci - its the first thing I did and good job too, it was down 10mm in diameter because of soot/carbon deposits.
may just need a good thrashing, especially if you're a careful town driver. if this doesn't help you may consider stripping things down and checking for dirty sensors/ clogs. possibly lambadas and CAT for blockages?
Thats what I would do anyway :-)
Best of luck, please keep it updated. Perhaps someone else can give you some more things to try :-)