i have a 56 plate 2.2 tdci transit that is doing my head in

it will stall at junctions or in traffic it pulls fine higher up the revs and when driving normaly

if i stop the engine runs for a couple of seconds then the revs drop to approx 500rpm and there is no response from the throttle if i turn off the van then start it up straight away it is fine until i stop again has slowly got worse did it once or twice earlier this year but today it has done it every time i stopped

anyone else had this or any suggestions before i start throwing money at it