Turning on and off repeatedly?

My phone has been turning on and off repeatedly. Not power off but lock the screen, though, the power button doesn't seem to be stuck or pushed down at all? It'll lock by itself and go off and on a million times. When I try to unlock my phone it'll take a million screenshots (again, the power button doesn't seem to be stuck). This has happened before and we took it into Batteries Plus and they didn't do much, it just suddenly stopped. Now that it's happening again, I don't know if it'll stop again. Help?