Looks to me like the slide is slightly out of battery when it stops shooting, this would engage the disconnector and prevent the hammer from falling.

Have you had the blowback cylinder off yet ?? Inside the cylinder is a seperate peice that sits on a spring in the port to the chamber and opens and closes as the pressure changes in the cylinder. If there is a bb in the cylinder it is bouncing around and occassionally getting stuck under this it will prevent the slide from locking up and the disconnecter will be engaged.