iPhone 4 stuck at Apple logo loop

Hi Guys, just a bit of background information to help. I bought the phone off ebay with a broken screen with the intention of replacing it and giving the phone to a friend.

Prior to the repair the phone would boot to the setup screen but only if plugged in. If not plugged in it would not turn on! Having replaced the screen and the battery to see if that would solve the problem the phone still doesn't turn on unless plugged in.

I can turn it on while plugged in and it will stay on the Apple logo for about 5-10 seconds before rebooting and the process repeating its self.

As far as I am aware it is not Jailbroken however the internal water damage stickers are red!

Any help will be greatly appreciated.

Thanks :D