G7 15 7588 lagging game and again running smoothly

Hello I am really sorry for my bad English but what my problem is that. I brought this laptop a month ago everything was fine every game was running buttery smooth. But suddenly after a month later I notice that my games runs smooth at first and lags suddenly and after a few seconds it again runs smoothly. This happens frequently every game runs smooth and lags, runs smooth and lags. Please help !!!