I had the same problem, and i eventually had to uninstall S2US (that, along with some other bugs that turned me off to the app)

I have my phone to just turn off the backlight when connected to an external power source since i use My Mobiler at work, and get sick of the constant disconnect/connect whenever it was powering off.

Once i uninstalled S2U2 the problem ceased to exist. All i have to do is hit the power button to put it to sleep, and it doesn't wake back up until the charge complete notification.

Hope this helps....