Well I was having similar instability issues with the new drivers too, but not always. I did some tests, uninstalling CCC and sweeping drivers between tests.
Here's what I found:
(2x 4850s Xfire) + (CCC 11.6) = Super unstable
(1x 4850) + (CCC 11.6) = Still unstable, but not as bad
(1x 4850) + (CCC 11.5) = Stable
(1x 5770) + (CCC 11.5) = Stable
(1x 5770) + (CCC 11.6) = Stable
So I concluded that the 11.5 drivers are stable across the board, but the 11.6 drivers are not stable with either/both of my XFX 4850 1GB cards.
I have nice Zalman coolers on both 4850s.. so I don't think the issue is heat related. I think the newest drivers (11.6) are just not jiving with my particular system components.
I wonder if we have other components in common? Mobo maybe? I've got the MSI 870-G45