when you get the black screen, press the volume rocker down and if it goes into vibrate, that means its your battery.
I had the same problem, the only solution was to do a battery pull. Took it to AT&T device support center, they reset everything, but still didnt solve anything. I got a black screen 2 hours after I left the place.
I called AT&T and they said they think its battery issue, and they will ship me an original battery and charger for free (phone is still under manufactures warraty, and I also have AT&T insurance). They shipped it to me and said they will call 1 week later to see how I'm doing.
1 week later: It was working fine, no more black screens but I wanted to new phone because my screen orientation has been off since day 1, and my live wallpapers didnt work. I told them I was still getting the black screen, and they sent me a replacement phone. The phone came without a battery or battery cover. I'm assuming it was refurbished.