Overheating possible causes;
A low coolant level, a coolant leak (through internal or external leaks), poor heat conductivity inside the engine because of accumulated deposits in the water jackets, a defective thermostat that doesn't open or does not open fully, poor airflow through the radiator, a slipping fan clutch, an inoperative electric cooling fan, a collapsed lower radiator hose, an eroded or loose water pump impeller, a defective radiator cap, or incorrect coolant mixture (not at recommended 50/50 water and anti-freeze).
* Cooling system leaks -- Loss of coolant because of a coolant leak is probably the most common cause of engine overheating. Possible leak points include hoses, the radiator, heater core, water pump, thermostat housing, head gasket, freeze plugs, automatic transmission oil cooler, cylinder head(s) and block.
Make a careful visual inspection of the entire cooling system, and then PRESSURE TEST the cooling system and radiator cap. A pressure test will reveal internal leaks such as seepage past the head gasket as well as cracks in the head or block. A good system should hold 12 to 15 psi for 15 minutes or more with no loss in pressure. If it leaks pressure, there is an internal coolant leak (most likely a bad head gasket but possibly also a cracked cylinder or engine block).
It is important to pressure test the radiator cap, too, because a weak cap (or one with too low a pressure rating will not allow the cooling system pressure to properly build to prevent overheating.