I had exactly the same problem... I think it's the Ram. However, just check well for viruses and spyware etc... use some Utilities (I suggest TuneUp) and clean your registry etc... if the problem remains, just start testing with hardware, remove one by one ram sticks, and try to change their place too. And think it's the VGA card (which I think it isn't), if you have another one exchange them, and see what happens.
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