This just started happening to me 2 days ago. Everytime I even slightly touch the lock key on the side of the phone, the screen either blinks off, or the phone powers off altogether (as it would if I were to hold the button down to shut it off). It's getting frustrating, as I have missed several text messages/calls due to my phone shutting off without my knowledge. Is anyone else having this problem?
Sorry for the inconvenience. I would suggest you to visit our Sprint Service and Repair Center to have your phone checked. It may be hardware issue( lock key on the side of the phone) or other option would be is Sprint Store can backup your data(contacts)and perform full reset on the phone.
Samsung takes all product related issues seriously. We invite the customer to contact us so that they can examine the phone to determine the cause of this incident. If you send us your name, phone number, phone IMEI, where the product was purchased, and email address to Social_Support_Mobile@sta.samsung.com, we will contact you to see how we can help.