Google Nexus 5 :: Why Is Phone Crashing

Around a year ago, my Nexus 5 kept 'crashing' on a very regular basis. However, I'm saying 'crashing' between '' because the phone was still on. I felt it vibrating when I'm called, the music kept playing and the notification light flashes. So the device was on, the backlight was working, but my screen remained black. The only solution was to 'hard reset' the device (holding the power button +- 10 seconds). The problem however kept reappearing.

I send it back twice, did multiple factory resets (with and without backup) and it seemed fixed. Until now, the problem is back since last week.

Sometimes it 'crashes' twice in a row, sometimes the device works fine for another 24 hours straight. I'm currently doing another factory reset, but my experience tells me this won't really solve anything. This is also the reason why it's hard to explain this problem to the manufacturer: when I send it to them, I lose my phone for more than 3 weeks minimum and it could be that their tests show no problems at all.