If it is caused by vibration, the best way is to balance the props very well, and track them so tip#1 and tip#2 pass through the same place instead of one above the other. Gyro noise filter (GCS -> Configuration -> Stabilization ->
Expert page) is last resort because increasing it is like running slower ESCs, you need lower PIDs to avoid oscillation.
You really
don't want to change the RC failsafe to "hold what it currently has forever". If you give it throttle and it goes out of radio range it will "fly away".
Also, if you switch the transmitter off first, it can start the motors up and you don't expect it.
Maybe just for a test to see if this is what is causing it, you could set RC failsafe to neutral sticks, but less throttle stick than hover position on full fresh battery, then it will come down slower. After running tests, it is best to put the RC back to "no pulses" because coming down with motors running can cut things like people and property. Motors running get caught in grass and damage motors, ESCs and cause fires.