Difficult to say, it could be a lot of things. Perhaps the system is having a hard time keeping itself cool? That has happened to every computer I've ever gamed on. Eventually all the heat and the fan usage takes its toll on the system, it has more and more difficulty cooling itself, and you have to decide whether to put up with it, get it repaired, or buy a new machine. On a MacBook Pro you should periodically clean the dust out of the fan exhaust and air intake vents, but besides that taking it to an Apple Store is your only maintenance option for the cooling system.