2002 Citroen C5 loss of power

I have had my C5 diesel 2.2.hdi for about 5years, not a bad car, but now I am getting intermittant loss of power especially when starting the car or when decelerating towards a junction or roundabout. I have white smoke coming from the exhaust when this happens and I have lots of messages on th screen, antipollution fault, top up fluids and engine overheating even though Ihave just started the car/.

Please help as I am not sure what I should do, sell the car or repair.

Regards

David