Overheating

At the age and mileage you listed the best suspects are the cooling fins on the radiator are corroded away, and a leaking cylinder head gasket. If the cooling fins are corroded, they will crumble like a rotten chocolate chip cookie when you rub your fingers over them. That will prevent the heat from being transferred to the surrounding air.

A leaking head gasket can allow combustion gases to pool under the thermostat which will prevent it from opening. Thermostats must be hit with hot liquid to open. Hot air wont do it. Your mechanic can perform a quick chemical test to see if the head gasket is leaking. That involves drawing air from the radiator, while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.

When the cooling fins are corroded, you'll typically see more overheating when the outside temperature is higher, and when driving at highway speeds. The overheating will be less noticeable at lower speeds and in colder weather.