Freezing up is a common problem with the S3, I tried all sorts of things from factory resets to full software reloads, in the end, I cured it by getting rid of my live wallpaper so I'm afraid that you're going to have to experiment all sorts of things, I would recommend that you uninstall an app at a time to see if it cures it.
Also, if you store your photos to your memory card and contacts to your sim / Google account, or even back up via kies, there's no reason why you'll lose it all.