3D objects overlap issue

I'm having a problem with 3D objects that are near each other in 3D space. I'm finding that any flat surface, whether it's an actual 3D object or 2D image, the back image starts to tear through the front plane around the edges when they are within about 0.2 units of each other. It only does it with the software renderer and never used to do this with versions previous to F8.

I've attached a screengrab of what I'm seeing and showing the gap between the two objects while still getting this corruption of the front image.

I'm wondering if it's some problem between Fusion and the gfx card (Quadro4000). Drivers are up to date.

... and I have just discovered that if I tick cull back face it seems to fix it!