Randomly my iMac says "You need to restart your computer." After it is restarted it says you have restarted your computer because of an error. Why is this happening, and how can i fix it?

Do you see a screen similar to the one in the link below? If you do you're experiencing kernel panics. These are often related to hardware problems, follow the steps in the links to isolating the problem.

http://support.apple.com/kb/HT1392?viewlocale=en_US