Hello, atm i'm using just integrated 9400 video chipset, if i enable those 2xnvidia 260m and start to play any game, browse some web pages or just keep my pc idle sooner or later will freeze if i'm browsing or start do some video artefacts on screen and after couple seconds will end with a black screen while laptop still works in background or even worst, freeze followed by a black screen instant shutdown.
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Upgrade your GPU driver or respectivally use the old once let me know if it worked
If that won't work please upload your game log or dxdiag log
I had a similar thing happen with a computer that used only one video card. Turned out the card had burned up. I swapped to the onboard system one day and it came back to life. I just needed to replace the card. Hope thats not the case!