I've had a few cases like that. The screen would intermittently turn on and off as if I operated the slider switch next to the headphone socket. In my hand in the middle of typing a message, in the pocket, on the table... Most annoying when it happens in the pocket since you unknowingly end up pressing something.

A reboot did not solve it. Neither did a full power cycle.

Eventually I've traced it down to moisture. It always tended to happen after things like taking a shower while the phone was on a shelf in the bathroom, getting caught in the rain with the phone in the pocket of wet jeans, that kind of stuff. Once dried out it would stop misbehaving again.

So I am more careful with exposing the phone to moisture and I've been accident free for over a year now.