My computer is like this all the time even playing Call of Duty and Dota 2 no to mention. Its because at some parts gameplay is heavy like sudden clash where heroes spam skill or whatever. The best thing you can do is get yourself a better laptop seriously... Nothing much you can do. My laptop (laggy fps) is an i5 and GT540M and my other computer is a i7 and Intel HD graphics which instantly solves the problem.

If you really insist on playing video games, save some money and get a new computer, its worth it...