Lumpy idle ?

My car E46 330d (2001) has covered 160,000 miles and has always ran well but recently has developed a very lumpy idle after engine is fully heated up, it almost sounds like the car is missing, as soon as you gently rev up from idle the lumpness goes away and car runs as normal, did once notice it did this fault when i had air con turned on but as soon as this was turned of car was fine. Any help would be gladly received