Run Furmark and see if your card artifacts at stock clocks.
http://downloads.guru3d.com/FurMark-1.8.2_d2523.html
If your card does artifact and starts having issues, do you still have warranty with it? If not, I would suggest trying to "bake" you card in the oven. I am serious.
http://www.overclock.net/graphics-ca...-oven-fix.html
http://hardforum.com/showthread.php?t=1421792