Yeah if you're seeing these artifacts very likely the solder balls under the gpu have cracked. If so then it will need to be removed & reballed (with lead solder). Or a reflow might work. I picked up a $30 W7000 that had vertical lines on the screen and my mining rig wasn't happy with it, so I reflowed the gpu and so far so good after a week solid mining...see how long it lasts.
Basically the same issue that plagued the early versions of the xbox 360
