check radiator is not blocked common to be over looked.

Water pump impellar broken.

Thermostat sticking.

Is the heater very hot?

A failed head gasket will not of itself cause overheating but what it does do is pressurise the cooling system and blow the water out - then it wil overheat. If you are loosing coolant that could be the problem, check around the filler cap and drain pipe for signs of dry antifreeze.