The first thing to do is determine if it is really overheating. Your mechanic can connect a scanner to view live data during a test drive. One thing that will really help with these frustrating intermittent problems is when you know exactly how to make it act up. The scanner will show the engine coolant temperature which we would expect to be at least 210 degrees if the fan turns on. At that temperature the dash gauge will still be reading in the middle which is normal. If the reading on the scanner doesn't go very much higher when the dash gauge does, I'd suspect a problem with the sending unit for the gauge or its wiring. If the temperature really does go up a lot and agrees with the gauge, have the mechanic test for a leaking cylinder head gasket. That involves drawing air from the radiator through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If the gasket is leaking the liquid will turn bright yellow.