so from the moment I've had this phone this issue has plagued my existence, sometimes when I'm using my phone it will randomly lock over and over for a minute or so, but not to the usual lock screen. the lock screen will change to a black background with a round clock, with a spinning light going in a circle counting intervals of 60 seconds. it sometimes dose this in the middle of calls and I have to wait for the person to hang up. pressing the lock/unlock button doesn't work and if it doesn't go away I usually have to do a simulated battery pull. I've just finished a 12 hour shift and had to ask my boss's boss to hang up because I couldn't work my phone.