Phone keeps turning on and back off. What do I do?
Try this take out your battery
Wait for 10 seconds
Re Insert the battery without pressing the power button 
if it turns on itself (witout pressing the power button( then it would be hardware problem. Just clean it with alcohol or tinner and keep pressing on it to clean the inside of your power swith or if not just replace your power button. 
If not auto turn on after inserting the battery then its software problem. Just download the official firmware ang flash your phone via odin.