I have a Jenn Air Microwave Model JMV9169BAP installed over my gas range and vented outside. It was installed about a year ago. It worked great up until three months ago, when the exhaust fan vent at the top of the micro started opening and closing constantly while the exhaust fan was on. When it stopped doing that it also stopped venting. The exhaust fan runs, but does not vent any air. Holding a candle or a napkin up to the exhaust vent does nothing. The napkin is not pulled up. Also the filter screens are clean and have not been cleaned in three months. Any ideas as to what the problem could be????????