Power button screen shot

I have had the same problem and in doing some research I figured out if you hold down the back button and hit the power button it will take a screen shot. I have tried this out numerous times and it works every time. Now that i know how to do it (on purpose) I'm aware of it and make sure I'm not pressing any other buttons while putting my phone "to sleep" and I haven't had the problem anymore.
Hope this helps!