Excessive phlegm when sleeping

My wife has been suffering periodically from an excess of phlegm in her throat, a problem that seems to be exacerbated when sleeping. Christmas morning she woke up gasping for air, and drinking the usual doses of hot water failed to get rid of the phlegm, necessitating an ambulance trip to the ER. She says that the plegm clogs her air passage making it impossible to breathe.

They found nothing of consequence at the hospital, only indicating that she shouldn't have eaten so close to bedtime.

Ever since she's been sleeping at night on the downstairs recliner, ostensibly to keep phlegm out of her windpipe.

Every time she has one of these attacks, she spends nearly a week incessantly clearing her throat, until the problem more of less resolves itself, although even in the best of times the throat clearing occurs multiple times a day.

She is not an outdoor person, and we have a hepa filtration system for our house. Neither of us can see any correlation between her outside excursions and the mucous problem.

She has no sinus infections, and I can't remember the last time she had a cold.

Any thoughts?