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I am using ubuntu 12.04 as my operating system currently.
Recently, I updated my system using the following commands:-

    sudo apt-get update
    sudo apt-get upgrade  

After updating,my system, first of all when I play movies then the screen starts flickering after regular intervals of time, flickering stops when I close VLC media player.
Also, after updating, my display has become a bit sharper(it's good that it has become sharper.).

Few minutes back, when I was watching movie, the screen turned pink, when I hard rebooted the system, a error message came that OS is unable to determine the display for the screen, I have to manually select it, after that I rebooted again the message didn't came this time.

How can I stop that flickering and what is the cause of this error message and flickering.
It's really annoying.
Has it got to do with my graphics driver?

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you might have broken your system using nonofficial repo. –  kenn Jan 17 at 17:19

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