Copter can fly stablized in Althold mode ,but oscillating in Flowhold mode.Already finished optical flow test
This part of wiki has good information about this problem
https://ardupilot.org/copter/docs/flowhold-mode.html
Warning
With many vehicles this mode results in wobbling and unstable position hold. If possible we recommend attaching a rangefinder and using regular Loiter instead
loiter mode must use GPS moduleļ¼so have to use FLOWhold