iPhone 6 getting gray screen

Yesterday for the first time my iphone 6 got an unresponsive grey screen. You can barely make out the apple logo in the background like the phone was resetting itself but it wasnt, it was just stuck there.

I tried many things. I resetted it by turning it off, I could barely see the area but I tried. I wasnt sure if it turn off. I did a hard reset by holding the power and the home button key at the same time. Nothing happen.

I decided to plug in the phone and left it alone. Came back a few hours later and phone was back to normal. Right after that I received the call from Apple care. I told them what happned and that phone was back to normal. They said it was probably a software issue. iOs 8.4.1

This morning it happened again. I pluged it in and a few hours later it cam back to normal again. I am tking it to the apple store on Friday, but has anyone have this problem and how was it solved?