After Effects :: Get Renders To Work In The Background?

I installed After Effects pretty much without changing anything, so the settings are more or less default. I'm using a Macbook Pro with OS 10.6.8 and After Effects CS6
 
I have enabled media and disk cache, max disc cache is at 100GB and I have an external drive (with 900GB spare) as the volume selected beneath that option. My internal SSD has not much spare on it. I previously had the internal SSD as the disc cache and still had this same problem.

My project is 1920 x 1080 25fps - And I'm trying to render to Quicktime. I've tried rendering to stills and then outputting to Quicktime (so as to have less computational effort), then running them through but the same problem persists.
 
If After Effects is the application in focus the project renders fine.
 
If After Effects is not the application in focus - ie. I work with it rendering in the background, the 'FINISHED' sound plays and the render is cut short a short while after I started working in another program.
 
The project render time is about an hour, so it's not really convenient.