I have a similar problem with my KJW P226, but it was heavily modded by a previous owner.
With standard KJW magazines, it will fire 6-8 shots then fail to cycle or vent gas as you describe.
With TM magazines, it will fire 12-14 shots quite well, but no more than that. My "work-around" is that I only run the TM mags, and I only load 12-13 bbs in the mag.
A friend suggested, like Rainy Eyes above, that the recoil spring may be too strong for the application and to try a less strong spring. I do get much higher FPS with my P226 than my other pistols, so I imagine that has to do with the spring and other modifications that the previous owner made.
I think I may invest in a pistol recoil spring that's a little lighter and see what a difference that might make.