What is wrong with my iPhone?

I have had this iPhone 5 for a while now, and I have not dropped it or cracked it and it has always had a case. However, all of a sudden the screen becomes unresponsive and vertical and horizontal lines appear. The weird thing is that when I lock it then unlock it, the lines disappear and I'm able to type for like a minute before it becomes unresponsive. Anther weird thing is if it goes less than let's say 50% of battery, this is when it starts to happen. Otherwise the phone is perfect. Does anyone have any idea why this happens?