This sounds more like an air flow problem than anything else. Quite a bit of times with your situation air from the engine compartmnent (very hot air) has know where to go and gets sucked back into the radiator. This air will not cool you engine since it is already hot. Make sure there is no gap from the radiator forward that will allow engine compartment air to go back through the radiator. Also make sure you fan is sealed tight against the radiator, if not it will suck air between the rad and fan bypassing the rad fins all together.