RE: heater blows cold at idle
It can also be a water pump issue, flowing sufficiently only at cruise RPM.
Do you notice any temp gauge issues at all? Fluctuation, inoperable, or even mild overheating?
Have you checked all of your hoses for resiliency? Collapsed hoses do weird things sometimes, but most of the time they just burst.
It does sound like air though, especially if there are no other symptoms. A radiator cap is an inexpensive possible solution.
One other thing to check: (Heaven Forbid!) Look for air bubbles in the radiator with the cap off while idling. Head gaskets and cracks can let compressed air into the system from the cylinders.
Fellow Chrysler Owner