I am having GPS problems on my Lumia 928 after the 8.1 update and tried hard resetting 2 times and 2 times using Lumia software recovery but did not go back to 8.0. I will try that next time.
I called Verizon support who called in Nokia Support but my phone is out of warranty (more than a year) and thus they washed off their hands by saying I am out of warranty!!!
So bad of Verizon and Nokia, it was the SW update that caused it and they should support!