Random chest pains that the doctor cant find a source from.?

Ive seen a few other questions like mine here, but I have went to the doctor quite a few times. Got all kinds of tests done, and he just couldn't find out what it was from. I have had them for like 3 years. They come at random times. It can be when im sitting there, talking to someone. When im walking down the street, when im cutting the grass. Just about any time. Happens from once a month to even 3-4 times a week. Like I said, completely random. What it is is a stabbing pain. I cant really pin point the exact place, but its around the center of my chest, maybe to the left a little. It lasts anywhere from 1-10 seconds, but then sometimes afterward, I can not take anything more then short breaths or it will hurt again until I breath out. About me. Im 26 years old. 5'8'' 180lbs. MAybe a little over weight, but im in ok shape. I smoke, maybe 1 or 2 cigarettes a day, but don't do drugs or drink. Can anyone help me with this? Im really at a loss.