Does it look something like this? My brother and I had an issue like this two or three months ago...had an old HD4670 in the system before we upgraded to the 6870. We needed a new Motherboard and PSU so we upgraded those, and the issue soon went away. (We have an XFX Double D HD6870)
However, I do not think you have a defective card. Try reinstalling Windows and see if that helps. Don't quote me on this, but it MIGHT have to do something with drivers...I can't be sure since I can't find any information about this error. But we're using a fresh install of Windows with the same card and it works flawlessly.