1994 Nissan Altima Overheats

I am new at this. I have search through a lot of old postings but to no avail. I have a 94 Nissan Altima with over 100K miles. As soon as I start the car air bubbles into the overflow. I have bled the system run the system to push out the air. Car over heats only when driving. Car heater works after topping off system and running in park but is usually the first to stop working. Car is loosing fluid. I can not find where this fluid is going. There are no visible leaks other than the steam and bubbles in the overflow. I have run the car with out a thermostat and then replaced with new thermostat. Replaced water pump. Disconnected every hose and run water through the system to check for blockage to include the heater core and heater open/shut valve. No visible drips from heater core. Replaced radiator cap. Pulled each plug. With one plug out at a time ran car to check for vapor in the combustion chamber. None. There is no water in the oil. Maybe, maybe some white smoke and vapor coming from exhaust but I think it is because I am looking too hard for something that is not there. Car runs fine until it overheats and then the fluid level is low. The overflow does suck back into system when system cools. I have searched other postings but can not find anything that sounds like this. Where is the air in the system coming from? If there is a breach in the combustion chamber what should I look for?

Ed