Must be a common problem, cause you are right on the money... ... I fixed it yesterday and it was the recirculation door that was broke.....
I found my problem..... Puilled the blower motor down and there was a piece of plastic laying inside the fan craddle..... Looked up and there my recirculation door was just laying there... It broke at the pivot point..... Last couple of days prior to this my recycled air would take forever to switch over from fresh to recycled or vise versa... . So i have to buy a new door..... they gave me the door for FREE... THANKS