Stuck in boot loop and itunes error 21, I've tried everythinng

Hi to all,

If anyone can help with this i'be forever grateful.

Bought an iphone 5 with a damaged screen hoping for an easy fix. Phone won't turn on without power, fires up when power is connected but is stuck in a boot loop, if i remove the power, it shuts off immediately. Its recognized in itunes, but stalls at "waiting or iphone" and gets error 21 when trying to restore in DFU mode.

I have replaced the battery and dock assembly but to no avail. Had a look over the board as well and no signs of damage or water.

Does this sound like a hardware or software issue? I wondered if maybe it has been jailbroken before and had a corrupt app installed?

I have tried everything I can think of: trying to enter safe mode, refreshing host file, latest itunes, different computer/usb cable, accessing hard drive with ifunbox/ieplorer but they cannot detect iphone....

Is this a lost cause?