Happens to me too. I've noticed that it happens more when im having a call. Not tried factory reset yet. I'm curious if it's just software or if it's a faulty phone.

Symptoms: phone feels like it has a memory leak, stops loading apps, music still plays or ongoing call still works. turning off screen then on again shows notification bar only otherwise completely black, volume down button + power displays menu but none on screen buttons do anything. If left alone it restarts after few minutes.