Hello, I am Dr. Andrea Russo and I will try to answer your questions. Chronic intermittent coughing in small breed dogs can sometimes be caused by a condition called "collapsing trachea" - it is common in your breed. It sometimes is worse after pulling on a leash if you use a neck collar. It can also act up at other times without obvious cause. Dogs with this condition have a flattened trachea, instead of a firm u-shaped trachea. It sometimes "collapses" or flattens and cuts off the airway temporarily causing coughing. It can be mild or severe. It sounds like your dog has a mild case, would you agree? I say that because it doesn't happen all the time. This is no great treatment for this but most dogs live with it without treatment. I would recommend a chest harness instead of a neck collar when walking him. Sometimes we will try cough medications.
Well, sometimes I will prescribe people cough medication but some medications can be dangerous for dogs to take. It would be best to ask your local vet to prescribe something. You are very welcome. Feel free to ask any more questions - I'll be away from my computer - maybe not back until tomorrow.