Any suggestions out there?
I am having the same issue. Found this thread while searching for a solution. Vibrations do not appear to be an issue in my case.
I am going to be equally unhelpful as Ken - I have exactly the same issue. Mine will actually land eventually.
I am having a same issue also. Did you solve this problem?
If you did. Let me know how did you do. pls.
Similar behavior sometimes happened to my quad (F450 + APM 2.6 +6M gps ) . I notice that when yawing in one direction until heading exactly north, the quad suddenly lost its height 2-4 meters. If yawing in the opposite the quad will gain height 2-4 metered too. This usually happened after the copter crash, So I fixed and performed a big alignment and calibration ( balance the propellers, tightening the gps rod), This behavior never appeared again. So I believe that it should be from the vibration Somrit
My very limited understanding is that asymmetrical quads (not + or X) just don't yaw very well. I'm sure someone can give you a more detailed description of the physics involved but the lack of symmetry just doesn't play well with fast yaw input. You can mitigate the effects best by adjusting the CG and optimizing the PID settings of the quad. I know of the effect through research and similar troubles with my discovery. The biggest change for me was adjusting the CG to recommended point.
Please keep in mind that I am pretty new to flying so take my advice with a grain of salt. However I believe that what I outlined above should at least solve the problem partly.