Just to be sure, the holding part (arms) are rotating (tilting) on vibration. As I posted earlier, my fix for that and the arm flex was to rest the tablet or one of the lower arms on the dash. Because I prefer the tablet to be slightly pointed at me, I rest one of the lower arms on the dash (the other hangs in the air). That works for smooth roads. Rough roads I end up with it pointing straight back with both lower arms resting on the dash. Over time both positions need to be scooted back. My phone has a smaller X mounted to my pillar, it will rotate off center if my hand bumps it from the steering wheel. At one time, I tried running it horizontal to use for my mapping. That would tilt on rough roads also.
BTW which mount are you using
Universal
Other Universal
The first one is the type I use on my phone, it moves more than the second type which I use on my tablet.