My touch id works for a short time when starting up the device. It will then fail and any attempts to add fingerprints shows the message "unable to complete touch id set up" How do I solve this? Thanks...

2 week old Iphone 5s and the touch id is already playing up. It will work shortly and then nothing. Attempts to add fingerprints again show the message "unable to complete touch id set-up" How do i solve this and make touch id fully functional? My phone will also randomly lock itself and attempts to unlock it won't work... Ir will remain on the black screen and re-open after around 5 mins to the power button slider. Any ideas?? Thanks...