probably linked to your particulate filter in your exhaust system which is a common problem with these,one thing you could try is to give it a good hard run in low gear but high revs to get the exhaust really hot to burn off those carbon deposits in the system,if this fails then it may mean a trip to a garage with the correct code reader to re generate the filter and also maybe top up your cerine reservoir,but be aware this can be expensive,the pleasures of owning French cars!
cdc, July 2012