im in the same boat. ive had had a couple of probs with my 96 DT. It was loosing coolant and overheating, i thought it was one and the same problem, but it looks like it is two seperate probs.
It ran fine on the motorway, temp gauge stayed in the middle. but when i started to go onto A roads, or up steep hills, or get down below 30mph, it started to overheat. I could always tell when it was going to overheat, as the air into the cabin suddenly went cold. I could temporarily fix the overheating by dropping it down a gear, and revving it for all its worth, that brought the temp gauge down again.
I have just been through the same illimination process. Ive had a new head gasket and heads re-skimmed. That has sorted the coolant loss, but is still overheating after being ran on the motorway. so it is now starting to look like viscous fan / radiator / water pump.
How do i tell if i have the old / new type of radiator?
cheers