I have a Honda Civic LX 2002 and I am having issues with my car overheating

I have a Honda Civic LX 2002 and I am having issues with my car overheating. My temperature gauge fluctuates, when I am at a complete stop my car will start to overheat but once I accelerate my car starts to cool down. Now, my first instinct was to change the thermostat, which I did but it's still giving me the same issues. I have check to see if the radiator or any of the hoses were leaking because everytime i drive i have to add coolant to the raditor but nothing, nothing leaks out. I don't know where to go from here, and I am getting tired of adding coolant to the radiator every single time I have to drive. Any suggestions?

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