[ecomodded]

If it is bubbling air then it may well be the head gasket is leaking / blown.
Check the oil for water contamination,milky oil.
If it is actually overheating it could be the water pump or a air leak in the system, having a pressure leak somewhere will cause the coolant to boil.
If the rad cap was faulty or more likely not done up tightly it would overheat. If the thermostat stuck closed it would overheat and the top rad tube would not have any coolant in it. If the water pump went it may not have water in the rad hose either..
First check if the rad hoses are both getting hot, a stuck thermostat would only heat one hose.

When you say water bubbling it makes me think that the head gasket is the problem, your temp gauges are saying its not the temperature overheating. Strange your check engine light has not come on.