A common cause for periodic coughing is roundworms. The worms migrate from the intestine as microfilliae thru the blood stream to the lungs. When the worms mature they migrate up the trachea (causing a reflex choke/cough), and then down the esophagus thru the stomach and back to the intestines. That completes the life cycle of the worm. So a first good try would be to consider worming the dog.

Another common cause is heart abnormalities, including the presence of heartworm. Other problems include tracheal collaspe/weakness, chronic bronchitis be it bacterial in nature or aseptic.

Your Vet will probably want to do the following:

1. Run a heartworm test.

2. Check for roundworms, and/or treat for roundworms. Sometimes a cough suppressent and/or short term corticosteroid treatment is needed to relieve any inflamation of the passages.

If the above doesn't solve the problem then ...

3. Treat with antibiotics, and a corticosteroid on the assumption it's a bronchitis condition.

Good luck.