In my particular case, the phone was not in any type of heat. During the 2 days that I had it, it was in air conditioning. I hard reset it and the problem didn't reoccur. Thing that bothered me is when the apple store said the phone registered it hit 130 degrees. Just picked up the replacement phone 2 hours ago. Will keep my fingers crossed on this one.
I had the same problem with my first 4G and my new one doesn't trip as much...
read here
https://forums.macrumors.com/threads/950997/