Your going to have a few more details than that my friend.
1) What make is your card. What brand/model
2) What sort of case and power supply do you have. Brand / model / wattage output
3) What values did you overclock to core mhz/memory speed mhz
4) What was your 3d mark score originally, and what is it now with the o/c ?
It may be that you've o/c'd it too much and your over the limit, and now it isn't working efficiently so it slows down.
It may also be thermal throttling as the overclocking made the card too hot, so it's slowing down to try and stop your card from burning out *literally*.
Lastly.... not ruling out the obvious, it may be that you using the different version of 3dmark now. 3dmark05 and 06 looks the same but you cannot compare the scores as 06 has a lot more features that make the score lower than 05.
Matthew