I just reinstalled WOW TLK and I got this problem with graphics. On most of the NPCs (only them, not environment)polygons flash (appear and disappear very fast) and usually the mostly disappear resulting like this :
http://img269.imageshack.us/img269/3176/problempw.jpg
I tryed different video settings and still. I played WOW TLK before on the same PC and it didn't bother me with such problem. Also changed setting from video card (Vsync and such) and still no change although I lean to think this is the problem).