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
You should overclock in smaller increments of 10 or 20Mhz also when it is unstable if you can raise the voltage that helps stabilize it but it definitely makes cards run much hotter.