Its not that its overclocked but its overclocked without the rest of your script being set up accordingly.
Post you GPU specs and your CGminer scrypt and I or someone else will try to assist.
An unstable overclock alone can give you errors. OP try lowering your intensity, also post whatever post your cgminer settings that you are using.
i did reset my clock settings, and tried to run with GUIminer now ia m having 220 khash and site say it same. i think i fixed it o.0 but i wanted to sue it clocked :S