Does the fan come back on if the ac is shut off? Are you in automatic mode or maual fan speed control? If you're in automatic then the temp sensors could be plugged. They're located behind the grab handles above the front doors. Pry off the plastic cap with the hole and CAREFULLY vacuum or blow out any dust. There's a fragile thermistor device inside the opening that's easy to damage and also a small fan that sucks air in thru the hole. My fan got dirty and it sounded like a hugh bee inside the headliner. Really odd!!! Don't know if the vent fan has any sort of thermal switch but if it does it could be shutting the fan off if it gets too hot.