[ubuntu] Ubuntu 13.04 - sometimes can't make screenshot

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http://ubuntuforums.org – Hello, I'm using ubuntu 13.04 x64 with unity. I have installed bumblebee support for my nvidia gf310m card. However when I try to make a screenshot (doesn't matter - shutter, default gnome utility, or even my own application in qt) sometimes i get blank file. When I get blank file once I must restart my system because I will be unable to make a proper screenshot. It's unrelated to screen capturing software. Thank you for any help. (: (Hardware)