Ubuntu does not shutdown

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http://askubuntu.com – Sometimes(rather many times), when I shutdown using the menu option >> Shutdown, the system hangs at a Purple screen with dots. I have waited for 10 mins, but then it becomes clear that i need to do a hard shutdown using power button. I am using Ubuntu 64 bit AMD edition(12.04.1 LTS) on HP Proliant Microserver(N40L) with OEM hardware. (HowTos)