Boot loop after screen replacement

I have repaired many iphone 4 screens and have not come across this problem. I changed the screen assembly and got the phone back together and working. It seemed fine when I handed it off to the customer. Wifi was good, cellular had signal, digitizer was operating fine. I had no indication of a problem until the next day. The customer brought the phone back and said that it just shut off and only would come on if plugged in. I took a look to find that when plugged in it would boot to the ios6 greeting screen as if it had been reset but it would not move past that screen. Cellular signal was stuck at searching (witch I realize is irrelevant at this point). I disassembled and checked connections and for visible damage and found nothing. I did notice a burnt smell inside the phone but no other evidence of burning (heat,smoke, black areas on board or housing). now that I got it back together it only boots to apple and cuts off before it completes boot process then comes back on. any advise is much appreciated.