Heating issue

[scottish_don]

I have a facelift V6 and having some interior heating issues. The engine goes to midway and stays theer and all looks good, however I only get hot air in the car when the engine is being rev'd. When it goes back to idle I get cold air in the car??

I had the car serviced and replaced the water pump and the thermostat at the same time (and all of the belts).

Any ideas??