Used to experience the same. Eventually had tonsils and adenoids out. Some improvement in frequency, but still used to succumb. Went low-carb (properly) and found myself without _any_ congestion, which solved the initial and subsequent problems. N=1, etc. One paper by Paton from 50's relevant (re pneumonia), otherwise nothing I can find in the lit. Recommended, though, for that and other readily apparent reasons.