Yaw issue with PH2.1 AC 3.5


I have an enough big octocopter (about 7kg) with a Pixhawk 2.1 with the GPS (from profi CNC). I have the following issue with the YAW. When I push the stick (yaw) on the right the drone turns counterclockwise around the z axis, but when the stick comes back in the neutral position the drone continues to turn counterclockwise. This phenomenon is so big that it’s very difficult to fly. The heading of the drone is controlled by the compass and I send the log (https://drive.google.com/file/d/0B_3d8P47v_RVVFRGZnk0NDdLMXc/view?usp=sharing). I’m not an expert to analyse may be someone is a guru in this field!
Thanks you in advance for your help