Alright so a couple months back my car started overheating. Cold air out of heater when on full heat setting. I changed the thermostat and coolant. Problem fixed. A month or so later it started overheating again. I thought it was a bad thermostat so i changed it again. Problem fixed. A month or so later it did the same thing. I figured somehow maybe air was in the system so i burped the system and it fixed it. Every once in a while i have to do the same thing. So somehow air is getting in my system? Its not loosing and coolant. Not much at least. Ive checked the outside of my engine multiple times everywhere and no sign of any leaks. None. Usually it starts with the heater not working as it should (no cold air on hot setting) and then the temp fluctuates. No leaks. No smoke out of tailpipe either. No oil and coolant mix. What could be the issue? Ive talked to many car people about it and they seem baffled by it. ????