Black screen problem

I recently started having a problem where my screen would go black while I play games.  Sometimes I could play for a few minutes, other times an hour or more.  The screen would just turn black, and the power light on my monitor would go from white to yellow, as if the system were off.  I can still hear the audio and the lights and fans from the system are still on.  This has only ever happened while I play games.I also noticed some time after this started happening that the LED on the Power button on my motherboard was staying on even after the computer shuts down and if I unplug the system.  I read about this problem and found that if I unplug my computer and then hit the power button, the LED will go out.  So I did this and it worked, and I left my computer unplugged all night, and when I when to play some games today, I have found that I have been able to play several times for a few hours without the screen going black.  I am not yet positive that the black screen problem has gone away, as there has never been any particular predictable time when the screen would go black. 

Thanks for the reply Alex.  I have dusted off my system recently.  I have monitored the temps and when I am playing they do not get much higher than 70C.  I ran the memory diagnostic and it found no errors.  Does the diagnostic built in to WIN 7 test all 6 GB...I was reading that earlier versions only tested 4 GB.  If the problem occurs again, I will read in to how to remove my overclock.  I was able to play for many hours yesterday with no issues after unplugging my computer and then pressing the power button to make sure the LED on the motherboard goes off, and then leaving the machine unplugged untill I used it again.  I cannot say for sure that those two issues are related, but dealing with one seems to have dealt with the other for now.