Indigo problem

I’ve been rendering some glass cubes for a while now, but a problem cropped up. Every other cube in each layer seems to have non-transparent faces on the outside of the main 10 x 10 x 10 cube. Sorry there’s no direct link to the picture, the image was just a bit too large for imageshack. 2 MB The image also seems a bit dark to me, although I rendered a brighter image without artifacts before (see below), I can’t remember what was different about that setup.

Hello? Anybody out there?..:smiley: