2012 US Spec 5.0 LR4 with front heater working fine but rear blows hot/cold intermittently.

What could be the cause?

 

Could be many things, check rear heater lines after engine's warmed up, if they're too hot to touch, then there's an issue with the rear climate unit, blend door, valve, etc. If they don't get too hot to touch, then there's an engine coolant issue or air in the lines.