Phone keeps rebooting itself every 5 seconds
I think you have to hard reset your phone in this way


Turn the Asus PadFone device off.
Keep holding the “power key ” and “volume down key ” simultaneously until you will see ASUS logo and the device will then go to recovery menu mode.
Select Factory reset/wipe data with  “volume down key”.


Note : If this doesn't work then may be it is some mother-board related problem

Hope this will help you out...