Hello. Sorry to hear that many people have suffered from this exact issue. I too had this problem 2 weeks ago and it turns out it is in fact a hardware issue. Specifically related to the power button. Luckily mine was still under warranty so i was able to rma it and get it repaired. Sadly the repair company thought that it was necessary to update the software (even though it was on the latest version of lollipop) as well as correcting the hardware defect so i ended up losing all of my data photos everything that wasn't backed up.
Bottom line is that this is a hardware defect people.