Problem... no walls

[Korinu]

I have a strange problem. I didn't find anything on the forum so i post it in a new thread.

The problem is that the walls just pop in when i walk nearby, before that they are just missing.

Here a screenshot:


After i walk further:


Hope someone can help me with that problem

-- Edit
My system:
Windows Vista Ultimate 32bit
AMD Athlon 64 X2 4800+
NVIDIA Geforce 8800GTX 768MB
Creative X-Fi Soundcard