I was having some issues with my game going horribly slow. Loading would take forever especially in dungeons and especially the fact the other players did not appear either for at least a good minute every new area that I entered which made my job as a healer very unbearable. Not to mention I'd randomly fall through the ground when trying to traverse the map.
You may need to replace your hard drive. From what you mentioned, it looks like the problem comes from the data being read from your drive to load the textures and causing the stutters or even failing to load it normally.

Another thing, do you notice any gameplay connection lag?