The heating acts as a supplementary cooling system.

Clearly more heat is being produced than the cooling system can dissipate - the harder the engine works the more fuel is consumed and approximately 60% of that is turned to heat (diesel) though fortunately most of that is expelled through the exhaust.

Either there is insufficient coolant flow through the radiator or there is insufficient cooling air flow through the radiator, considering the age of the vehicle it is likely to be a combination of both.

Sometimes a modern waterpump with a plastic impeller can lose a vane or sediment can build up in parts with a slower coolant flow or sometimes a bottom hose can become soft and tend to collapse at sustained high speed but most likely the radiator has become tired and both the flow of coolant and air is reduced.
When replacing a radiator it can often be sensible to have a recore by a local radiator specialist who can usually uprate the unit as part of the repair.