Videocard hickup. Happens to myself occasionally too.

You'll notice maybe a second of freezing, game will continue, but if it tried to load any textures during that short period, you'll have to either logout/login, or change resolution to fullscreen/windowed and back again to reset it. Mildly anoying, but it's just a driver issue i've been told.

Makes for some interesting psychedelic screenshots atleast: What was in those Rolanberry's!