The idea that it works when driving and then not as well when you stop leads me to believe there is air in the system.
I attached directions below for static filling/bleeding the system. Do this when the engine is cold.
Let me know if it helps. Also, it starts on step three because steps one and two are telling you to close the radiator drain and engine block drain.
Let me know if it helps or if you have questions. Also, let me know if the engine is reaching operating temperature and if there are any other issues with the cooling system.
Take care,
Joe
See pic below.
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Sunday, February 19th, 2023 AT 9:50 PM