Pixhawk cube flight mode random switch

I have a problem with my pixhawk cube 2.1 AP when I prepared the the drone to fly with mission planner and transfer from RC flight mode (manual) to Auto mode the drone will try to follow the WPs I have introduced in mission planner then suddenly we could hear the engine voice was changed (throttle is increased) . when I downloaded flight data I found that the flight mode is turned randomly with out any action by me from Auto mode to manual mode without any command from RC on the ground for fractions of second then it returned back to auto mode
I have set up the RC to double action switch but the problem is not solved
Is there any solution for this fault as I am afraid that I will lose my drone when it is out of RC signal Range