Rib/Chest Pain

I am 16 yrs old. I have had this problem ever since i was about 7, although that is not precise. I get these random pains in my chest area. at the bottom of my rib cage, at the left and right side of the sternum and at the sides of my chest area. Sometimes i wake up with them, other times i am talking to someone and suddenly i feel an excruciating pain in the chest area. Some are stabbing, others are not and they last for various amounts of time and are unpredictable.When i breathe deeply in, it sometimes hurts right at the bottom of my lungs. I have been to the doctor twice now, with a space of three years in-between them, and neither have been productive. The first time i was told they would go away, and they have not. Each side of my sternum is tender apparently. Is it something to do with my muscles spasming? intercostal muscle issues? All i want is for these pains to go away. Please help. Thankyou.