It happens to me and others i know sometimes.. only when its really crowded tho.. or there are lots of mobs about.

I think it has something to do with how many models your computer can render or your settings.. im not too computer savvy tho heh..

Easy way to fix it is run a ways out 50 yalms or sight distance.. a grid square works fine also if you aren't sure on distances.. then back again and it should load.