Black Screen

Hello sorry if there is already a thread explains it (I am new here I only found an old one similar to this) but once in a while a face a problem with my screen which turns black...
It does not go off since the task bar is still visible but everything else is just black for a few seconds (usually after having my pc running for a few hours)

I have an in i3 3.2GHz cpu, 4GB RAM, power supply 600W and a shapfire Radeon HD 6570 gpu
And I have Win7 64bit

I have this pc for over a year playing games never had a problem before even with my previous power supply witch was 450W so I guess power supply is not the problem

Is it possible to be an overheat problem that can be solved with another fan?
Or even the Win7 witch I had re-installed because of new HHD?

Thanks in advance

Ps. Excuse me if my Engish are not so good if something is not clear I can try to rephrase
 
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