I have my gimbal tilt control / pitch at 90 degrees so that I can shoot straight down with my GoPro.
At home, on the ground I can tilt the gimbal to 90 degrees and it's fine.. But in flight, when I tilt to 90 degrees it starts to shake and wobble and I have to tilt it up a bit to say 80 degrees to stop it wobbling.
It may be due to momentum of inertia is changed when the camera changes orientation. Try to re-tune PIDs when camera is 90 degree down, and use it for last