I've had this problem at times in the past and have been told by doctors that it is asthmatic bronchitis. The symptoms are similar to bronchitis, only they are caused by asthma, and the cough is a dry cough instead of a wet one. I found inhalers such as Aerobid to help. When this has happened to me, it was usually during the fall allergy season, so allergies probably have something to do with it. I started taking allergy shots about five years ago to try to get over it, and ever since then, I haven't had a problem. When I used to have it, I would only have to use the inhaler for a couple of weeks and the cough would go away once the allergy season passed. I always looked forward to good, hard freezes and snow falls to take the allergens out of the air. Saline sprays such as Ocean are good to rinse out the nasal passages, too. One year after finding out how much better I was doing after a trip to a warm Caribbean Island during winter, I started putting a pot of warm water on the stove with sea salt in it, and breathing it as I went by. There's something about breathing warm sea (or salt) air that helps.