Help me!CO2 leaks and Sputtering but only w/ paint?

[takeastand]

I just got a rebel vertical and the first time i played with it, i started having trouble. The gun works fine when shooting just Co2 but when shooting paint, it begins to sputter and fails to re-cock. my Co2 was full and the gun was oiled. There also seems to be gas leaking from the barrel area sometimes. I'm getting an expansion chamber soon but what else might help this problem. please help me.
thanks

[Creek]

If you not shooting the marker it is most of the time the cup seal and or valve seals. But you said some of the time so I'd say cup seal. Now if its only doing this with paint in the marker farting etc. It kinda sound like the o-ring on the hammer is blowing by. Which isn't letting the hammer slam all the way back pass the shear. These are like a few maybes here. With out standing right with you i can only give you a few things it might be.