Hello, can you tell me if the cooling fan is coming on when this occurs? Doest the temp. gauge go up? Is the coolant at the proper level inside the radiator, and the over flow? Is this a 3.7?
I forgot, the only cap is on the bottle. It sounds like either air/combustion chamber gas is being introduced and vapor locking the system or the water pump is not flowing the coolant.
I need to know when it gets hot and the fan kicks on, do you have continuous hot air coming off the rad. I have seen where the impeller for the water pump justs spins on the shaft, and does not circulate the coolant.
A chemical block test can be done to see if combustion chamber gases are in the cooling system. The tester sits on the overflow and the chemical turns blue to yellow if there is presence of hydrocarbons.
I hope this helped if so an accept is appreciated
If they are present you have a bad head gasket or a cracked cylinder head. http://www.monstronix.com/uvu560000-p-61560.html
There is also a proceedure to pump up the cylinders with air through the spark plug holes and look for bubbles in the overflow.
I hope this helped if so an accept is apprciated