Like mentioned, you would want a better cooler.
I've read some very good things about the ThermalTake Volcano 7+ (Read what O/C has to say about it:
http://www.overclockers.com/articles539/ )
The AVC Sun Flower wouldn't be a bad choice either.
Grab some Arctic Silver II as well, and apply a paper thin layer between the core and the heatsink. It should do you some good.
Try raising the voltage a bit as well, just a little at a time.
If none of this helps, let us know, and we will do our best to help you further.