[juicy]

Potentially, a valve knocker/valve problem? I'm thinking its likely something to do with the seals/valves on your mag; do you have a spare magazine that was not left out in the cold? Try shooting with that magazine, and see if there's any noticeable difference. Aw hell, I can't tell you much without taking a peek at the internals.

I'm not an expert on this, nor am I claiming to be. In fact, I should probably just leave this to a more experienced gun doc.

Take it to a local GBB doc!!!

Edit: never mind about trying another mag. I just read your original post again... yeaaaaaaaah, that sucks. Posting pics won't get much done, I do seriously suggest you take it to a gun doc for at least a diagnosis.