Suggest Treatment For Throat Irritation Leading To Cough

Hi, I have question regarding regarding persistent cough that my mother (68 years) is suffering from. Symptoms: Throat irritation leading to coughing (with little or no sputum) occurring sporadically throughout the day specially while sleeping. Chest x-rays showed that her lungs are clean. She was put on medication twice. First instance she was administered Ivepred and Telekast along with Betadine gargle by a doctor. The coughing stopped within 2 days. But it restarted after 2 weeks. Again I took her to the same doctor, he then prescribed Medrol, Telekast and Betadine gargle to continue. But this time also after medication the cough went away but it again restarted. Any suggestion as to what may be the problem.