My Macbook Randomly Restarts Itself

I'm not so sure if this can be reversed by doing something in the System Preferences, but as far as I've tinkered around that area, I can't seem to find a way to stop my Macbook from restarting itself after leaving it on its own for around half an hour or so.

I've read that there are numerous problems about Macbooks shutting down on it's own; I don't think that's the problem with my Macbook - it just restarts and restarts if I don't use it for a certain amount of time. Is there any way for me to make it stop doing that? It's kind of annoying when I'm downloading from iTunes or when I'm playing music then it restarts out of the blue and completely stops all the programs that are running.

Any suggestions? Or should I bring my Mac to an Apple Station for repair?