Alt Hold motors dropping to zero

Hi, I have just completed another build, a Alien 680 quad frame. I did my maiden flight today.

I flew in stabilize mode most of the time, but tried Alt Hold twice. Both times the motors went to basically zero and the quad dropped from about 4m. Looking at the logs it looks like as soon as I flick the switch the THR_OUT drops to 130 then slowly increases. THR_MID is set at 500 so I am unsure why THR_OUT is dropping so low. Specially as the DALT is always above ALT so I would have expected it to increase throttle from the THR_MID point.

I noticed the vibrations are high so I need to sort this out, but this shouldn’t affect altitude? I wont to understand why it is doing this before I try again.

Attached image shows the CTUN message at the second Alt Hold switch.

Sorry the BIN is 39MB so I’ve had to load to a Google Drive; … sp=sharing

Any input into why it is behaving like this would be much appreciated.

If vibrations are high, APM can behave in odd manner. I have similar problem like you, after putting in ALtHold, copter behaves abnormally. But even after I revert back to stabilize mode, it fly on its own and yesterday it crashed.