Check you settings for email if you are not using a push service such as @me or Microsoft Exchange. Make sure it is set to fetch the mail on manuel otherwise it will drain the battery quick. My only other suggestion is to do a full restore and not off back up you might have a memory leak somewhere in the phone that is causing this to happen. I had this issue with a 1st gen iPhone. Also you might want to send it in to Apple as it might have a short somewhere in the battery or the soldering points might have come loose during heavy use. Have you been noticing it has gotten warm? that would be a sign if it has been getting extremely hot