Well, Android problems can't be guessed easily.
But, since you're new here, I'll give a quick fix / suggestion :
- See your power button, if your power button is broken / covered by dust, fix it.
- If you're an overclocker, set max clock to lower value.
- Remove / uninstall some apps (such as Antivirus app, or useless battery booster app)
- Remove unused script / tweaks
- Do a quick cleaning (wipe data / cache)
- Finally, if those quick fix don't work, change your battery (to a new original Samsung battery)