This is a classic start of head gasket failure symptom. The gasket starts to leak compression into the cooling system very very slowly, the air rises to the highgest point which is normaly the header tank where it is seperated from the water however, some bubbles make there way to the heater which results in an air lock. The air lock gets bigger and bigger untill the water pump is having trouble moving the water around to cool the engine. Assuming you have check the usual stuff i.e. water pump, thermostat, collapsed hose, blocked rad, faulty viscous unit ect.
regards
Dave