My PC freezes when I'm playing games

Hello,
A few months ago I've built my own PC, and since a week ago I'm having serious trouble with it when playing games.
My problem is that my PC freezes when I'm in-game, I don't get no blue/black screen just a frozen image of my game,
my keyboard and mouse stop with working(I can't press CTRL+ALT+DEL), I don't hear no sound from my headset/speakers.
A buddy of mine said that it is a RAM problem, so I ran a Memtest86+ test(Only 2passes, I was short on time) it didn't find any errors.
Keep in mind that this problem is only occuring when I'm playing games!

Please help me out, I spend all of my saving on this machine and it's not even working properly  
Thank you very much for trying to help me out!

My specs are :

*Motherboard : Gigabyte Z77M -D3H
*CPU : Intel i5 3550 (NOT OVERCLOCKED)
*GPU : Asus Radeon HD7850 (I did some tweaks with the software you get)
*PSU : OCZ ZS Series 650W
*HDD : Western Digital Caviar Green
*OS : Windows 7
*RAM : 2x4GB Corsair Vengeance CMZ8GX3M2A1600C9

My temps are :
CPU -> Idle : 31°
CPU -> In-game : 48°

GPU -> Idle : 28°
GPU -> In-game : 49°-52°

All of my drivers are up-to-date, it's defenitly not a driver problem because I've deleted and then installed all of my drivers.
(Except for my motherboard drivers, I used the CD that came with my mobo to install the drivers)