Object Mapping using an Empty - showing different results

Hello,
I am using blender for rendering architecture images. In order to have a texture aligned in all my model, I tend to use Object Mapping relating to an Empty. That way, I can control with precision regarding position and scale (e.g. 120cm) all the objects in the scene that use a certain material, instead of using UV UNwrap, which is more suitable for organic shapes.
The problem is that I am getting different results each time I render. First renders, it aligns correctly with the Empty I specified, but thereafter it shows inconsistent results.
Blender bug? Me bug?