First you could do a simple pressure check on your cooling system to make sure it is sound. Also even if it appears you have good flow through the radiator you could have damage to your water pump impeller vanes that you can't see that is affecting the volume of your pump output. Also your radiator cap. With the thermostat you want to pick the right one to keep the coolant in the radiator so it can do it's job and then open up and circulate the flow. And everything the other guys said. LOL