I had the same problem...was very worried, nothing showed up on any film, just this sharp stapping-like in the same place...after some time my doc told me it was most likely a bruised muscle (I don't remember the name) that is located just below the breast...on the left side. It turns out that after months of trying to figure this out, when my doc suggested a bruised muscle, I remembered that just prior to the pain beginning, I had fallen. I was carrying a hard plastic milk carton that was full of fire wood. As I approched the fireplace my cat got caught up between my feet, I went down hard. As I fell, my (what I thought at the time was a rib) hit the edge of the box, causing the bruise. no discoloration, only internal damage. It lasted for about 6 months...I also had the breathing problem, which was aggrivated by the fact that I am at an elevaion of 5500 ft in the mts. You will find that applying heat to the area will give you some relief, but talk to your doc. It's a tiny little muscle that comes out of the bottom of the heart...he can show it to you on a picture of the heart. It has no affect on the heart. It's just a stupid little muscle that will heal in time. Can you remember any kind of an injury to that area?