The power button of My iPhone 5 is not sensitive! Screen flashes, and brightness darkens.

There's some problem of my iPhone. The power button is not very sensitive. When I wake up the phone or make it sleep by pressing the button, it sometimes doesn't work. It happens several times in one day. The second problem of my phone is that when I wake it, the screen sometime just flashes in one second and then sleeps again. I don't know why. The last problem is the brightness. Auto-brightness is always off. But when I wake up my phone, the screen brightness sometimes is darker than what I have set. It is really strange, and I don't have solutions.