Hi,
I had one of these for a while. Are you using 12g capsules? If you are an issue can be that the capsules only just get pierced leading to very slow recovery from a shot and thus big variations. On mine I had to take a tiny bit off the bottom of the CO2 chamber, shortening it, so the CO2 piercing cap made a better hole in the capsule. Once I had done this is shot very well indeed.
Hope that helps.
Frog