I have an old (early 2009) iMac that has been running fine up until about a week ago. All of a sudden it will turn off randomly. Just completely shut down. When I turn it back on, it fires up fine... like nothing happened. The shut downs are just a complete shut down, as if someone pulled the power cord from it. It's instantly to a black screen. I don't change anything, press the power button, and it comes back. There don't seem to be any connections between what I'm doing and the machine turning off. Any thoughts on why or how to fix it??