We have had this problem when the alignment was not correct. What happens is that a cluster of cameras (for example 5 cameras) has a slightly incorrect alignment compared to the other cameras in the project (but a correct alignment compared to the cameras in the cluster). Because the alignment within this cluster of 5 cameras is correct, it still contributes points to the Dence Cloud, but they are offset compared to the rest. So parts of the dense cloud have an incorrect position, this leads to "stepping" errors in the Dense Cloud.
I am not sure how to fix this. You could try to run the alignment phase with higher settings (but when we had this issue we were already aligning at the highest possible settings. In our case it was because of slightly blurred images in the photo set).
You can also do a quick inspection of the photos using the free utility we release a while ago:
http://www.agisoft.com/forum/index.php?topic=4070.0