Have you burped the system? If air gets trapped it can prevent the thermostat from opening and make your engine overheat. To get the air out, start with a cold engine, turn the heat on vent or floor only and run full blast, run the engine with the radiator cap off until it gets to operating temperature. Keep an eye on the coolant level and add as needed until it doesn't drop in level for five minutes. Some cars may require removal of the thermostat so you can drill a small hole in it to allow air to escape from the engine side of the thermostat. Keep any eye on the temperature gauge to make sure you don't over heat it while trying to burp it.
That's true about the head gasket. If you leave the radiator cap on and start the engine cold and run it for a minute, then check to see if you can squeeze the radiator hose together. If the head gasket is busted it will pressurize the cooling system and make it very hard to squeeze that hose. Now, when the engine gets warm that hose is pressurized so it is important that you do this on a cold engine.
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You may also want to a compression check. it could be the head gasket.
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