Check for silt in the bottom of the Rad....when the engine is warm, feel the radiator and make sure it is hot all over....
The Diesels use a top flow Rad, with the ingress and egress of coolant at the top. Two things happen, the bottom gets silted up and also there is a baffle in the top chamber to direct the water in and down and not straight across the top...
To check baffle, remove Rad, turn upside down and install a marble into one of the apertures...if the marble rolls out the other side, baffle is dislodged and coolant will not be circulating the Radiator correctly!