Unknown crashes

I am having this unknown crash problem when hovering but only sometimes. I have done full missions before but about 2/10 flights result in a crash as if the motor/s lose power and start tumbling down. Here is the log file : https://drive.google.com/file/d/15J3tNBz5YqSBHfddGQIRICnn79sj2xAX/view?usp=sharing

Any help would be appreciated !

Have you got a .bin log file?

And please update to ArduCopter 4.0.3

Already answered on FB. Faulty motor2. Check cabling, replace ESC.