Those video glitches can occur if your video card is getting too hot. I had the same thing happen to me when I would BG or go into Dalaran because it has to work harder in those areas. Turning down shadow quality may fix it because shadows are a big usage of the card's power. I fixed my problem by buying a fan that goes into a free slot right next to the video card and blows right on it. I've haven't had any problems since.

Here's an example of the type of fan I used. They are very cheap, like $10-$20.
http://www.newegg.com/Product/Produc...82E16835119097