Truck starts to over heat when sitting or idling

there could be many things that could cause this from water pump not producing the proper flow, cloged radiator, but my one thing if I have know any thing is the clutch fan. these things are almost as worthless as bees at a ant farm. what they were designed for was to save fuel as you traviling down the freeway the fan works less saving you gas and in town it is supose to work more. I would guess that it is going out and there for you are not getting the amount of air that it needs to cool through the radiator. i would atleast start here there not that expensive but on this truck you need a special tool to get it off. if you do it your self make sure you look to see what way the threads are going if you use the tool and go the wrong way you might make things harder by tightening up the fan.