start with burping the system again.
check flow at upper and lower hoses , (hot or cold) heater core can plug up as well so ensure thats good. can possibly use air to blow it out. some guys use a bypass or gate valve thing in the heater hose to help with burping system and bypassing heater core all together, good to check stat as well have been bad out of a new box (can boil on stove to check if working). ensure no leaks and rad cap is good. also overflow tank should not be full only 1/3 full when good. needs room for the hot coolant to cool down.
Some look to a open system from a closed system after a while .