Slow performance on many games why?


Nov 25, 2012
iam notice a lag in many games and low fps and sometimes the game lagfor 2 sec then it will continu like {fast forward resume }
so what whould be the problem ?
my system is >

motherboard: gigabyte fxa 990 ud7
vga: sapphire hd 5970 2gb
cpu :amd phenom ii x6 1100T black edetion 3.7ghz
ram 8giga kingston ddr3 1600mhz
power supply :ezzcool 1050w 4 rail
screen: lg 1080p full hd 24inch

the games that i found lag with them hitman absolution , assassins creed 3 , battlefield 3 , crysis 2 ,and many other high end gaming

so what is the problem with my pc and tell me what i need to improve my pc so i can play all games on ultra high setting