I have issue with I7 4500u temp (DELL 15R-5537)

Hi I have 4500u when run something heavy, like games, after few minutes the frequency drop from 2.7-3.0 to the 1.8Ghz and its make game laggy. Max temp i have notice in such test around 72-76 degrees. My question why the temp stop at 76 .. it should go around 95-100°C.. and how can fix this problem.. thx...