My 2006 Xterra (70k miles) heater blows cold while idled, then heats back up when I start driving. Took it to dealer, they flushed cooling system...same problem. Took back to dealer then they did a back flush...1/2 way home...same problem. Jumped in Internet and someone said transcooler leak??? The temp gauge is normal and it moves to the 1/2 way point after a few minutes of starting the engine. I do not have any type of fluid leaking on my floor.
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If you had a coolant leak, you should see your coolant level drop. Low coolant flow at idle (like at a stop light) can cause the problem you describe.
What temperature has the engine been running at lately? If the thermostat is stuck, you may see symptoms like this. Also can happen if the water pump is not moving enough coolant at idle, but I would expect that to last longer than 70K miles
Air in the heater core can cause no heat problems, which is probably why they sold you the flush the first time. Not sure about the second time, might be good to speak with the service manager there to get more help.
This article may help explain the condition more:
http://repairpal.com/i-have-no-heat-what-could-be-wrong
