Feedback on the Lenovo ThinkPad P50 - Walter

Hi there, I am facing huge problems using dual-display configuration(external connected via DisplayPort) since I installed Ubuntu16.04.
I am trying to setup extended desktop over both panels. Laptop built-in is left one. Both panels(internal and external) native resolutions are 3840x2160 but want to set both to resolution 1920x1080.
My P50 boots correctly to login screen, where resolution is set correctly(both panels 1920x1080) but after entering credentials there are some unwanted but visible screen mode switching and all this finishes in one of few modes of unity desktop:
1.(most often) both panel set to 1920x1080 but some "x-zoom" or "x-panning" applied and mouse cursor acts as if it were on different position as it actually is. Roughly told mouse cursor active point is double as far from most-right position as it actually is visible, all on external(right) panel is displayed correctly, all on buit-in laptop pannel zoomed in x-direction 2times
      (interesting here is that this does not apply to "Logout..." and "Shutdown..." system dialogs. If mouse is over one of them, "position where I click" is same as visible mouse position)
2. laptop-builtin panel set to 1920x1080, external panel set to 3840x2160
1. both panels set to 3840x2160
4. few times it switched to different modes, but never to the correct one...

What could cause this troubles?