Ok if anyone is having this issue here is the solution:
1. Put your phone in fastboot mode (volume up + power)
2. Plug your usb cable in. Go to your PC and open the cmd prompt from the folder on your pc that has fastboot.exe (shift+right click in folder, select open command window here)
3. Type fastboot format cache
4. Then type fastboot reboot
Solved the hanging/boot loop.
silly me, i was always using:
fastboot erase cache