Note recommendations above. Verify you're not leaking coolant anywhere. Check all hoses, metal plumbing, radiator, and lower intake manifold. Assuming you have full coolant levels in the radiator/reservoir....
....Cooling system bleed @ the water pump might help. There's a small brass bleeder valve on the cross over pipe that runs into the water pump.
If the bleed corrects your problem, I would then try to figure out how air got in there in the first place.