iPhone 7 stuck in boot, still after dfu? works for an hour after

Hello, I had an iphone 7 at my store yesterday.. the phone showed apple logo then turns off and apple logo appears again. I put the phone in recovery and updated via itunes. it was complete and I got to the lock screen typed in password. then everything seemed to work as it should. Today they came back, phone was back in boot loop. I put it back in recovery mode and itunes resat the phone back to “new”.

Now about 2 hours later they called and its the same issue even tho I was playing on the phone for about 30 minutes before calling them saying everything was fine.

Anyone know or has an solution to what the issue could be?

I did try with a new battery as well, same issue.