Viscous fan, check the spring is in good condition and the valve is rotating, if it doesn't rotate properly and fully it doesn't let enough oil through to drive it at the appropriate speed.
Usually the spring is corroded and being a clock spring means it unwinds.

Viscous coupling lost its oil, not uncommon on most vehicles and you can strip them and refill them and you only need around 80 ML of silicone oil which is readily available from evil bay cheaply.

To do this you need to remove the viscous coupling, remove the fan, then split it in two carefully, refill with oil, refit the two halves back together, and rebuild.

Clean your cooling system, with vehicles of this age it is very common for them to have been neglected, usually by filling with any available water, this builds up internal corrosion which forms silt in the bottom of the block and this rises up the cylinder liners, hindering thermal transfer, good industrial cooling system cleaner. Do this twice if it is really bad.

Check the thermostat as some types have the wrong temperature, these are winter stats and some were marketed in the UK and fitted by many, and cause this problem. Basically they are slow to open so the engine gets hotter quicker, but don't open as much as a European market stat, thus impede flow.

Check the water pump impellor, these can corrode if the cooling system has been neglected and the lower surface area means less water is pumped round for a given engine RPM.

Check the radiator cap as several were supplied by the trade, this was a common issue with them, some were the wrong pressure and some had a defective seal on the radiator cap which prevented it sealing correctly, basically the seal was too wide and caught and sealed on the radius of the radiator filler, thus lowering the working pressure.

Right: where to begin? forget flushing with water as that moves very little, if the cooling system is in such a state you need to do it properly, and it takes a little time.

Set the heater to the hot position in the cab, don't forget this or the heater matrix will not be cleaned, and it can air lock when you refill the system.

Get the engine hot and drain by removing the bottom radiator hose, be careful not to burn yourself doing this, the pressure will remove any loose crud.
Refill with clean water only, run until hot again and drain by removing the radiator bottom hose, repeat this until the cooling water comes out clean.

Go to your local hardware shop and get a 500g bottle of Vanish oxi action, drain the system again and put this in, top up with water and run until it is warm and stop and leave it for a couple of hours, run up to temperature again and leave overnight. This is an excellent cooling system cleaner and will loosen most cooling system crud, run up to temperature and drain, you will see a lot of crud coming out so run a hosepipe through any cooling system hose you can remove to flush the system of the Vanish, fill with water and run, drain, refill, run, and drain.

Get a good industrial cooling system cleaner, Serck Marston do an excellent one, but FOLLOW THE INSTRUCTIONS ON THIS TO THE LETTER as this shifts anything, leave it in too long and it will eat through gaskets and even the engine block, drain and flush several times with neat water to remove every trace of the cleaner.

Fill the system with 5 litres of anti-freeze, top up with water and job done.
To improve your cooling system, if you have a Brita filter jug or similar, filter a quantity of water, boil in the kettle and put in a bottle in the fridge to cool, use this instead of tap water as filtering and boiling removes all the impurities.