I have a really weird issue with certain textures (flashing textures and blanks)

First of all, I would like to say that this happens only with some textures, not all of them.

I have placed a texture on the landscape ground and when I open the first person preview and look right where the character’s feet are supposed to be, the texture starts flashing and blinking. There are also some weird blanks between textures, since it’s not just one stretched texture but more of them.

Screenshot of the blanks: http://prntscr.com/i1mjcq
Video of the texture flashing and blanks: https://youtu.be/mortCOgo-Gw?t=14s (flashing starts at the time in the URL, you can watch from start for the blanks)

I did build the level after I placed the texture, just added some other stuff after that. I had the same issue with the textures right after building.

Why does this happen with certain textures and how can I fix it?

Thanks in advantage.