If the bbs are diving, it could be because of a couple problems. From what you have described it is probably due to the o ring coming out during it, but here are some other things to make sure of.
1. The bucking has to be aligned a certain way. It cannot just be placed on the barrel and then attached to the chamber. There should be a small slit along the bucking that needs to align with a slit in the barrel. When placing the barrel/bucking onto the chamber, the small protrusion on the inside of the bucking must be placed up facing the small open gap in the PDI chamber. This is where the nubbin goes over top, and the arms put pressure down on the nubbin.
If this is not done correctly, the bb's, when fired, will result in massive HOP DOWN. Even when you adjust the hop up screws for more hop, it will do nothing. Instead, dry firing and low hop shots will result.
It is hard for me to describe the small o ring installation, but I figured it out by reading through this one post will relatively good pictures. There was a method involving the end of a colored pencil (unsharpened side) to keep the o ring from sliding around, while using an allen wrench to fit the o rings in the grooves (inside the chamber).
Here is the link: http://www.airsoftforum.com/board/lofiversion/index.php/t102401.html