I it is boiling then you are loosing coolant through steam loss. The hole can be tiny. Trust me guys, I have been here twice. If you have not had the problem then you have no idea what you are talking about.
A faulty radiator cap will cause the problem. Why? Because a faulty radiatorcap lets air in which is what? Oh yeah, A LEAK!!!
Before you take that tone with people you may want to ensure you know what you're talking about.
As AndyAK said, a bad cap doesn't maintain pressure which lowers the boiling point of the coolant. The temperature of vaporization of any liquid increases with the pressure.
Secondly, it's not air bubbling out of the system - it's water vapor -steam. When the engine stops the coolant is left in contact with the hotter portions of the engine, it boils locally. As the liquid changes to gas, its volume increases hundreds fold. The steam rises to the top and is pushed out to the reservoir along with some coolant.
Anything presented here is conjecture. It's foolish to think you absolutely know what the problem is when you can't see the problem and have only scant facts. From what a person says, there is the possibility to exclude causes, and experience can suggest causes, nothing more. I’m asserting that the bad radiator cap suggestion is a good suggestion. Not the absolute cause to the problem, a possible cause.
Stargazer said that when the engine is warm and he stops the engine it boils like mad for about 10 minutes. He says nothing of coolant loss and a leak would suggest.
The questions I would ask next is:
1: Is there a coolant loss?
2: I the coolant a proper mix of anti-freeze and water?
3: What does the temperature gauge say before you shut the engine down?