If you set Kodi's GUI to 1080p, it will stay in 1080p and will play media in 1080p.
What it might also do is change framerate to match your video material. Is this what you are seeing?
If so, and it is proving a problem, then set your gui framerate to be that same as the material you play the most often - ie if you mostly watch 24 fps movies, set the GUI to 24 fps - if you mainly watch 25p/50i TV then set your GUI to 50, etc