Here are the points to ponder as I see them...
Is the water pump in good condition? Inadaquate coolant flow.
Is the radiator cooling adaquatly? Plugged, either in the outer cooling area or inside the core.
The one most often the problem, corrosion in the block. The block has developed a layer of corrosion which is blocking heat transfere. Sometimes a good "flush" will help, but often complete engine dissassemly is required for hot tanking. The problem comes within a few days of filling a cooling system completely with water without an antirust additive.