I got the exact same problem and it happens everywhere very rarely and very HARD to reproduce, but it can essentially appear anywhere on screen had it since January, I built my PC at work (We make PCs) so I done every possible test we could, we still are testing it today (Full system) and we cannot reproduce the issue it's just random, I have posted this issue everywhere,
http://www.tomshardware.co.uk/answers/id-2162298/260x-artifacts.html including AMD forums, Sapphire, Sevenforums etc. People just say the same thing, Now I am convinced since you got the same GC twice and you're getting same issue as mine, I'd say it's a driver problem and AMD are sucking to fix it..
Plus I also got the Sapphire 2GB OC edition so it must be a driver issue..