6s Plus keeps crashing. Screen turns white with apple logo, then changes to pixilated grey screen, then to black and repeats this over and over. It won't let me shut it down. Have already restored to factory settings and it stopped for only a month.

6s Plus keeps crashing. Screen randomly turns white with apple logo, then changes to pixilated grey screen, then to black screen and repeats this over and over. It won't let me shut it down. After about 30 minutes, it finally goes to my home screen and works fine. This will happen randomly throughout the day and usually happens when I click the power button to activate and use the phone. Have already restored to factory settings with apple care support and it stopped for about a month. But it just started doing it again.