I would bet that this is a [http://rockitrepairs.com/cell-phone-repair/power-button-repair/|power button repair]. Most of the time this happens as the solder becomes unstable where the power button is soldered to the logic board.
Here is an image of a power button repair
[image|521179]
Here is a link to our power button repair video. https://www.youtube.com/watch?v=qXqPqAsfTOQ