if its a diesel this is not uncommon,i would recomend try putting some fuel treatment in the tank to clean out the injectors you can get this from places like halfords& use fully synthetic oil instead of mineral oil,another thing that can cause this is if you dont drive it hard enough it can cause a build up of soot in the exhaust,so try taking it for a blast down the motoway occasionally to blow it out,failing that check all the air hoses onto the turbo etc for splits.