HK KWA USP.45 gas issues

[cogz101]

I recently bought a second-hand USP.45 and am having issues with the gas. it will only fire about ten shots, then start missing shots, then after some more trigger pulls, all the gas will leak out at once. I know that it isn't the mag not working because I recently changed the valves and bought a new mag, both run out of gas at the same amount of shots. Any advice on how to fix this?