I had same problem 2 weeks ago, I dont know if its related but after i done update for iOS 8.1.2 about an hour later it shut down then booted back up and shut down again and just kept looping. I plugged in the charger but it just kept looping and switching off still so I took charger back out and left it looping till it died completely. By this point i got so frustrated I left it dead for 3 days, I dont know if it matters how long it was left off but I had a brainwave to plug the usb charger into my Wii and left it there for five minute and all of a sudden it woke up to the home screen and its been on ever since. Im no expert but i think leaving it off for a few days actually helped. I imagine if its stuck in a loop Then so too is the clock in which the boot system uses to start up but like i said im no expert but it seemed to work for me thankfully. Ive had similar problem with laptops and desktops stuck on a loop and Believe it or not the bios time showing wrong time and date was a big part of the problem when stuck in a loop. I hope this helps.