The game "lags" when its played for 30 m

Hello, my son noticed this problem, whenever he plays for 30 mins or more, the game starts getting slow and he cant even play.
he plays maple story and age of empires. this happens when playing these games.

my technician installed a program called mem turbo4, it shows the ram, but i dont even know what it is. the normal ram i have when using internet is 350-500 m ram free. when he plays, the ram goes down to 50-90 m ram free, i guess it has to do something with the slowed games.

if you guys need something more about the computer, just tell me.

i'd appreciate if you help me.

thanks.