I just have installed Ubuntu 12.04 64 bits just few days ago, and everything is ok, but just some times when I tray to shutdown the system just freezes for a long time, and I have to force the shutdown with the power button.
Any advise or any way to use the terminal to know what is the problem.
I'm using a Dell with ATI video card, 4GB Ram.
Looks like a possible duplicate. You should go here. And try to see if it solves your problem.
Digvijay