Samsung SPH-M610

I have owned the m610 since the first day the store had it and I love it! But, recently i have noticed that from time to time the external screen with not turn off until the phone is opened and closed at which point the internal screen stays on and the external screen stays off until the phone is restarted. As you can imagine, this is an enormous drain to the battery and I now have to send it to Samsung to get a new one. Any ideas of why this is occurring?