Computer Screen Blackout

Computer Specifications:

Intel Core 2 Duo Cpu E7500 @ 2.93 GHz
RAM 4GB with 2.50 usable
Windows 7 ultimate 32 bit
Nvidia GeForce 9600 GT 512MB

Hi guys. So recently I installed Steam and Counter Strike: Global Offensive onto my computer. The game's system requirements are as follows:

Intel Core 2 Duo E6600
RAM 1GB XP/2GB Vista
GPU 256 MB or more

Download was ok, every thing went smoothly. But after playing for about 30min+, the moniter suddenly went gray, as in light gray. The audio was stuck at a sound clip of the programme. I then rebooted the system and everything was fine. But only when i opened the programmed again, soon after the same thing happened. I have already tried reinstalling the game but still, same problem.

Before installation of Steam and Cs Go, everything worked fine. But now, when I try to reboot, sometimes the screen would just blackout and monitor displays no source detected, but cpu is still running. Logging in also takes really really long. Even when normal surfing of the internet, it sometimes blacksout and monitor shows no source detected.

I would like to ask, is this a problem of my graphics card lose connection, overheating, or the problem of the programme Steam. Help is much appreciated. Also any suggestions on what to do? Thanks alot.