1966 Mustang inline 6 sputters when hot

I have a 66 coupe inline 6 and when the temperature outside gets hot the engine runs hotter than normal. When the engine runs hotter than normal, the engine idle at a stop seems to lower to a sputter and you really have to feather the gas lightly to get going or else it will stall out. This problem only happens when the engine gets hotter than normal, and the engine only gets hotter when the temp outside the car gets hotter. Would a newer air cleaner help this?? or a fan shroud (since I don't have one on my car)??