A related discussion, Chronic Bronchitis was started.
You should have an exam of both your lower and your upper respiratory tract. This would include a laryngoscopy. This procedure is done using a small camera at the end of a flexible tube, called a fiberoptic laryngoscope. During this procedure your vocal cords and the upper part of your windpipe can be examined. You also need a thorough evaluation for fatigue. You might consider seeking a second opinion.
Your "periods of ill health" could be due to asthma along with an allergic condition called allergic bronchopulmonary aspergillosis (ABPA) causing the yellow mucus. However the yellow mucus would be more typical of gastroesophageal reflux disease (GERD). This is the back flow of stomach contents, including digestive juices, into the esophagus or food tube. This is most commonly called acid reflux or heartburn. This reflux can come all the way up into the mouth and nose. When this happens the material can be aspirated, dribbling into the windpipe and lungs. This could explain your symptoms, including the bronchitis.
GERD can cause the following symptoms:
I have had excessive mucous problems for years and it lead to constant infections, etc. Turned out to be food allergies to wheat/gluten/SOY. Felt better after the FIRST DAY avoiding them and have continued to improve - been a month now and feel like normal - had forgotten what it's like to breathe easily.