I have the same problem.
Did all the suggestions that I can find on the web but nothing works.
So, decided to send in for warranty.
Sent on July 15, 2015.
Called the service center today to find out the status of my warranty and diagnostics.
I was informed that they have to replace the whole motherboard and I have to wait for a month for the replacement part to arrive!
I reside in Malaysia. This could due to the long festive holiday that is going on in Malaysia.
Can't Microsoft adopts the wonderful practices of Apple by replacing users with a brand new or refurbished phones so that user do not have to wait for a month?
I did ask the service center why not they just give me a new phone instead of asking me to wait and the reply was that it is not the policy given.