Preview problem with UE4.8

Hi all,
i have a problem with the preview in UE 4.8. First i lost my direct light and when i build the lighting, everything was ok, but when it happed again and i did the same thing it went very bad. In the two screenshots you can see the problem, there are black lines and dots all over the project (the first one is in Unit mode and the second one is in Lit mode).
Do you have any ideas how can i fix it ?