Planes shake sometimes when you fly straight. I'm sure that you are aware of that issue

https://streamable.com/t4s4w7

You have too much lift or elevator area for that plane. It's a configuration issue, not an IV one. It's been like that since the MFS days, and happens when you take a plane past their designed physics. For example, the Trimotor does it around 30 blk/s, but the Eclipse only does it around 50.