Hard to say. It could be that they new drivers are stressing it in a way the old ones did not and thus exposing some minor instability in your overclocked card. Or, it could be that your card is just getting older and can no longer run at those speeds (that happened to my 6800s). If you go back to the 9.10 drivers, do the artifacts go away? If so, just stay there as most likely most of the improvements added are primarily for the 5800. Or it could just be that these drivers are buggy.