When Loiter mode motor would loss speed

I have had problems flying my drone on Loiter mode. I tried Alt Hold and STA on two situations, the motor speed is normal, but when using LOITER mode, one of the motors will always drop back to the idle speed.

I would like to ask if you have encountered such a problem and how to solve it.

Please follow these instructions:

And post a .bin file afterwords. No, that situation is not normal at all. But you do need to follow those instructions before we can help you.