Default Problem With HTC One PN071.

Hi All,
firstly i would like to say that the phone i am having an issue is the HTC One From Telstra in australia....
ok now we have that out of the way...... this phone is my fathers and he needs it for work so me being able to fix it is urgent....
afew weeks ago the phone downloaded an update over 4g and resulted in being corrupted in the download (my father prob drove into a low signal area as he is a truck driver) after this happened his phone started rebooting its self.... it would not get a network connection and it would make it as far as the lock screen and reboot and thats as far as it would go...... it fixed itself up somehow and booted up fine got a network connection and so on.... once i got to the home screen i got a message on screen saying that it attempted to do a software update but the download became corrupt (600+mb download) so i connected to wifi and tried again... 30 minutes later the update was installed and his phone was working good as new...... fast foward 2 weeks to this morning and his phone started playing up again.... this time u could get to the home screen and use it but it would reboot itself again in less than 5 minutes..... and during the time that it is running it isnt detecting the sim card.... ive tried his sim card in my galaxy note 3 and it works without an issue so the sim isnt the problem in any way shape or form... what i need to know is how can i fix this.....
here is what i am thinking of doing
backup his phone
find out how to restore the firmware through recovery (his phone is still under warrenty though so i dont want to do anything to void that)
the one problem i have is that i can not get a backup of this phone because it reboots itself..... any ideas??? i will love u guys forever if your able to help?