I have followed Alexander J Velicky's tutorial on how to create distant objects and terrain for a worldspace. But it only half works.

Terrain textures turn black after a while and the distant land is not shown properly.

Some objects also disappear (they are not visible even when I walk into them, though their collision is there). I used "cow" to reach a small town in my worldspace. Everything worked fine. Then I explored an areas further away. Upon returning to the town, all buildings were now invisible...

Any ideas what is causing all this?