Games lag horribly after 20 minutes or so

[Indigo Ace]

Ok, so I'm using an alienware mx18 with 2 1535MB GeForce GTX 460Ms and when playing any game it will work fine but after 20 minutes or so the game will start lagging horridly and i found the only way to fix it will to either put my computer in sleep mode for like 2 seconds or just unplug the power cord and it will work fine. the problem with those is that if I unplug the cord my battery will drain super fast, and if I go with the sleep mode, it will only work for another 20 mins before having the same problem

 

[Brett928S2]

Hi

Heat....

Clean it out with compressed air...

All the best Brett