Cod4 lagg problem

Hello! So my cod 4 just suddenly after 10 mins of normal no lag playing the ping gets high randomly and its even pretty impossible to quit the game itself and it couldnt be cuz of the shitty pc cuz on system req lab it showed recommended! So can any1 think of something that could be causing the lagg? And whats wierd is that after the pc hasnt been used for a few hours u can play normally w/o ping for alot longer time so rly dont know what could be the problem though thought about maybe the airfilter thing could be full of dust or? Just let me know here if u might now whats causing it and thanks!