Default [Q] LG G Pro 2 on AT&T.. Just shuts off!.

So after reading everyone's post. I got one off eBay. So I put my straight talk sim in there and the apn loads automatically!? Meaning I didn't put anything in a everything is working. LTE.. Voice, everything seems fine. Well then within 15 min, 3 huge system updates. So now here's the issue. If i let the phone sit for 5 min, and go to use it, it's off! Like it turned itself off. I have to power it on. Any idea's?