I have toyota land cruiser 2000 model and I am expired in overheating. The radiator has been washed and it is getting worse. What could be responsible for the problem

Toyota land cruisers use a viscous fan hub attached to the fan and the water pump
they have a habit of failing,resulting in over heating in situations like at lights , slow traffic and high rpms under load ( hilly work. cross country
the over heating cools down when highway speeds are attained
although it spinning when you look at it ,it will not drive and pull air through the radiator.