Copter 3.2.1 on APM sudden drop in Altitude in Alt-Hold up to a crash


Using APM from CX-20 (open source) version 2.52, encountered few drops in Alt-Hold mode - copter just drops out of the sky, I’ve tried analyzing the logs yet can’t make solid conclusion to what is wrong ?

Any help will be appreciated, logs attached.
2017-05-02 (3.0 MB)

Your vibration levels will be having a large impact on performance.
Have a look at
I would get those vibrations down a lot and then do an autotune