One of the problems I used to have with CO2 is that it was very inconsistant when it had just been filled. You need to give the gas time to settle in the bottle, whihc is why I used to have 2 bottles for my spyder. The expansion chamber worked perfectly well in most weather apart from the extreme cold. When you are playing with one bottle, have the other one already filled and settling down. That way, you can use the bottles on a rotation basis, and the CO2 will be fine. An anti-syphon is a must as well, and at the weekend, I would bet that some liquid CO2 got into the gun. That is always good for giving you inconsistant velocity. Hope you sort the problem out soon. Nothing worse than an inconsistant gun!