I am having exactly the same issue with my Sapphire r9 280x Toxic.
Card is on stock and i get random artifacts in WoW, BF4, League of Legends. They appear randomly after a while of playing. I can play for an hour without artifacts and then suddenly out of nowhere the artifacting starts. Restarting the game usually solves the problem.
Do you guys think this might be hardware or driver related?
Also I ran Furmark, 3Dmark and OCCT for a while without any problems/artifacts...