How is the airflow in your case? If the heat isn't taken out the case, then it will build up. This will result in a cpu getting hotter and hotter.
Well.. there was the air flow problem in my case but i have placed one 120 mm fan in the case to overcome this problem. And also my cabinet is always open. So i can't see the ventilation to be an issue here. It just the air in vicinity of heat sink which seems to be not moving or the cool air is immediately replaced by hot air coming from heat sink.