HD4850 - overclocking issue

[D749]

Threw in one of my two HD4850s tonight and ran a battery of tests while at stock (3DMark Vantage, FurMark, Lightsmark, etc.). I then cranked the card up to 700/1000 and re-ran the tests. No problems. Later I got adventurous and cranked the memory up to 1100 which was a bit too high (e.g., driver crash then black screen while running FurMark). A reboot got me back into Vista but for some reason I can't seem to get back to 700/1000 without having the driver crash (e.g., while running FurMark). I'm not sure why a previously stable clock would now be so unstable.

Any ideas why this would be? Could I have damaged the card somehow or missed a setting in CCC?

Thanks.