Why does van loose all power dramatically at 2500 rpm for several seconds b?

2 litre turodiesel van 2009. The engine has been loosing power black smoke comes out of exhaust after and then the power returns 15 seconds after but the van must be driven below 3000rpm so it means changing gear at low revs. if you accelerate between 2500rpm and 3000 rpm it looses power completely for 15 seconds with black smoke. its had brand new injectors and head gasket. what causes this is it the turbo. Any help would be very appreciated thanks merry xmas and happy new year everyone.