1999 Toyota Landcruiser Loss of power

I have lost some power as the car does not pick up speed as it used to. I get some smoke on start up and it does not start as well as it used to, I have had the injectors serviced by a local garage that specialises in diesel engines. When I tow our caravan the engine overheats after about 30 to 40 miles but driven without the caravan this does not happen the temperature stays at normal even with a heavy load in the car ( a 1000kg of sand ). I have no problems with the gearbox it changes when it should and the engine does not over rev when towing which would indicate a gearbox / torque converter problem. I suspect either the injector pump or the turbo, any advice would be appreciated.