Samsung S4 Boot loop issue - All options and troubleshooting exausted

My Samsung S4 SGH-I337 16GB is stuck in boot loop.  It has all firmware and app updates.  It was working fine in the morning.  I noticed a couple of hours later that I had not received any calls.  Checked and it was off.  I re-started it.  The Samung logo appears and it goes dark.  I removed the SD card and tried it with several batteries.  No luck.  I tried the Safe Mode and recovery.  It still turns off.  I can get the Android guy a few times, but it turns off afterwards.  Best Buy tech said he has seen this issue many times, but only with ATT phones.  Has to be software related.  Best Buy tech said generally, most phones will eventually boot once and he reflashes them and all is good. ATT is offering no help at this point.  After a year, I hate to pay $600 plus for a new phone?  I would have purchased insurance, however isn't that for physical or water damage?  Insurance shouldn't be to protect ATT from it's own software!!!  If anyone has any other ideas, I would appreciate it.  Thanks