GambitSunob

I would also lean toward a memory leak of some sort, or the re-creation of resources.

However, it could just be that the video card is slowing the frame rate down to 60 fps. Some video cards will do this as a way to save power. If that's the case you can verify it by monitoring the frame rate. It will have very distinct drops in frame rates and wont drop below 60 fps.

I've done some voxel-ish stuff with this process : http://www.visionsofafar.com/dablog/post/2011/04/27/Voxel-Modeling.aspx
Resulting in this - https://vimeo.com/23143112

That process should give you some decent 3D models which you could then render in an isometric view.

There's also - http://code.google.com/p/sproxel/ for an alternative Voxel creator