The screen refreshes every 20 seconds when i connect to the second monitor

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http://askubuntu.com – I am running Ubuntu 12.04 and use the LibreOffice Impress to do my presentation but when i connected the projector to my laptop. It was fine around 20 seconds after that every 20 seconds it gets refreshed and the projector and laptop screen go blank. After refreshing the display alternates between the full screen and file and blank screen every time. (HowTos)