ArduPilot's mysterious crash

Hello everyone! I have a problem with ArduPilot that haunting me. Basically the problem is that when I have low throttle (such as when descending) one of the motors stop intermittently. This causing quad to tilt at dangerous angles and several times even crashed.

This is video example of the crash Ardupilot Crash

0:20 the problem starts, but then flies normally. 0:40 problem intermittently repeats and causing crash.

Please share your experience and help me to find a problem in my setup because I am out of ideas as to why.

You might want to go over the ESC to motor solder joints, I recently had an incident with my quad ending in a crash… after going through the motors to clean them, I noticed that one of the bullet connectors had a bad solder joint that was broken that the bullet/wire joint, see my video… looks similar to what you experienced.

1:18 to 1:41