Adding onto Wolffox, if you're getting just a plain ground texture in some areas, and just in general weird textures, go onto your options tab, which can be accessed by pressing Esc, then options. Next, press the video tab and change the resolution.
If you're on Direct3D9, change it too OpenGL, and if you're on OpenGL, change it to Direct3D9. You'll have to close down FH, and you might spot a difference. :3