^Well opening a case isn't a good idea. Sometimes it will help but usually a closed case cools better because your air current is more concentrated, stronger, and hot air is sucked out better. If you have an open case, air won't be pushed through it as well and as fast and it can result in worse cooling. What case are you using?...
-If everything fails, buy new thermal paste, a new cooler (something different).
-And also, are you sure you don't have turbo or something like that enabled?...it overclocks it automatically, try disabling it.