are you using the stock cooler? normally they are decent with the C2d's, but yours might be a rare bad one. If your using a aftermarket, make sure its on tight, and you have decent amount (that is not too much, and not too little) of thermal paste.
note: this would involve needing to re-apply the paste after to re-created the original contact, so don't check that out yet if you don't have any arctic silver laying around. If it does happen to be a stock cooler, then this shoudln't be an issue.