In the first instance, try a factory reset (don't forget to backup your content and make sure you contacts are calendar are fully sync'd first). It would be part of any repair process, so you won't have lost anything.
When you set the phone up again, run it vanilla, with no extra apps, for a few days. If it's stable, add the apps back one at a time. If the issue recurs, the last app in is the problem.
If that doesn't do the trick, you'll need to send the phone off for repair, I'm afraid. You could also try taking the battery out and putting it on a flat surface. If you can rock it, it's dead and probably faulty. Contact Samsung, who are pretty good abotu providing replacements.
