This kind of behaviour usually occurs if you have issues with your graphics card or drivers. Is your libsdl package up to date? Do you really have all 32bit shared libs in place? Where did you get your NWN version from? Did you install the latest patch (1.69)? This is sometimes neccessary even if the installed version claims to be 1.69. Some people even reported it helped to reinstall NWN - which would suggest a faulty install of some kind.
Some download links for all the patches links can be found here, some hints on how to copy over an up to date working install from Windows/Wine to Linux can be found here. Be sure to read the first post on that thread. To check your install, you can use this guide.
HTH
Skildron