Volkswagen Sharan cuts out when slowing down

just bought my vw sharan and when i bought it i was told that sometimes it cuts out when it is idle and that the carborator need changing so i did that. it is still cutting out on me though when i am slowing down or when i am idle. the power steering stops and so does the brake when this happens. it starts again really easily but i can do a journey that takes half an hour and it can take about ten restarts to get to where i am going. i havent a clue what to do please help me