Hi ChiCotje,
Have you been able to find a solution to the shaking? My FSX is doing the exact same thing. I am using 3 displays. I went from two lower end GPUs, where it did run correctly on low settings, to a pair of 2G Radeon HD 7850s. MS Flight does the exact same thing as the FSX does, just like your video shows.
I became distaught and purchased X-Plane 10 Global. It works fine, and I can stretch the view across all 3 displays.
I really like FSX though. I still use FS2004 without any problems.
Let me know if you find a fix.
Thanks much,