Default [Q] Holding volume down required to do anything

Hello,
Have a very strange issue with my 2012 Nexus, it seems to have a bootloop issue happening where it starts up and after 5-10 seconds restarts, I see the white google logo & perhaps a small bit of the animation (the four circles one for 4.4). This happens if I go into the bootloader too (anywhere in fact)

Oddly I've found if I hold volume down constantly it goes away and the devices boots up but only into safe mode for some reason. I can get into the bootloader & TWRP etc, but I do have do constantly hold volume down to do anything, as soon as I let go I get 5-10 seconds and it restarts.

Currently the device will only turn on if I plug it in to any USB (PC or original 2V charger)

Any ideas or help would be appreciated, I can't see anyone else anywhere having the same issues & I'm not sure Google or Asus would take it back now.

oli