Problem is \"lock\" at 20 fps. (Dishonored 2)

I have a problem of following character: When enabled in the game settings vertical sync to 30Hz, the framerate is fixed at 20 frames per second. Very strange. When you turn sync on 60Hz or shut off everything is fine. The option to play at 60hz will not go because the framerate is not stable and often drops to 40-30 fps, it causes severe discomfort. Much nicer to play with a stable 30 frames. And without vertical sync do not want to play, the image of the breaks, and it is very evident, I have this option always in all games include. Thank you.