[ubuntu] Srange boot behaviour Ubuntu on iMac

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http://ubuntuforums.org – I have Ubuntu 12.04 installed on my Intel iMac. Except the keyboard everything is working fine out of the box. There is however one thing I like to be solved, that is strange startup behaviour. When I start the iMac Ubuntu will startup and failed to startup turn by turn. When it starts I see a underscore blinker for a few seconds afterwards the graphical startup screens is displayed and Ubuntu loads. The next time I start the iMac I only get a black screen. After I restart the iMac Ubuntu loads again. It is on a turn by turn basis. Strange is it not? Does anyone know what to do? (Hardware)