What Causes Mild Pain In Left Side Of Chest?

My husband is 82 years old and in good general health. On his left side on the upper part of his left breast he has some pain that goes in a zig zag. It is quick and does not last but a second or two. Sometimes it will happen several times. No dizziness, weakness or any other symptoms. His primary Doctor has checked him out good, x-rayed the area and nothing shows up. What could possibly be causing this?