First observe if the radiator fan turns on when the overheating occurs, especially if it only occurs at lower speeds. After that, the next suspect is a leaking cylinder head gasket. If combustion gases get into the cooling system, that will prevent the thermostat from opening. The clue is the coolant will not be flowing, so the radiator and cap will not be very hot.
Your mechanic can perform a chemical test at the radiator to check for a leaking head gasket. That involves drawing air from the radiator, while the engine is running, through a glass cylinder with two chambers with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.
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