Mine only did something like that after an app crashed. The screen went black, and I got no response from the phone. Had to pull the battery to get it to restart. But that only happened once.
The first thing I would do is think about what apps you have installed. Did this start after installing something? Even though the app in question might not be something you are starting up, it may still be loading into memory and causing problems. If anything comes to mind, uninstall it and see if that helps. If not, you can always try erasing all data from the phone and restoring it to factory defaults. That option can be found under Settings, Privacy.
If all that fails, you may have just gotten a dud.