Maybe your over flow tank is filling because the radiator cap is not holding, have the radiator cap pressure tested. As far as not overheating, something is going on that you're not seeing. Could it be you are filling the reservoir because it drains or leaks, but the radiator is still full, or is the cooling system drawing the coolant into the radiator. Have you checked the radiator to see if it is full. Could be the gauge is not working properly, or possibly it is overheating is why the coolant is going into the reservoir, but your gauge is not showing the proper temperature. Do you know someone with an inferred temp gun to verify the actual temperature?