S40 - Fuelling issue

Its a R plate S40 TD with 156000 on the clock. Whenever the engine is started from cold there is plumes of blue smoke from the exhaust and it is prone to stalling when stopping the car until the engine is warm. Even when driving if the revs drop below 2000 rpm their is a lack of power when the throttle is pressed and more blue smoke out of the exhaust. The only way to stop it is to bring in the turbo. A coupe of friends have told me it is a fuelling issue so i changed the fuel filter but it made no difference. On occasions when driving when the engine is cold it is very hard to get the turbo to kick in at all!! ADVICE NEEDED PLEASE!!