After driving for 30-40 minutes the air flow from vents goes from high velocity then down to a trickle. I can hear the fan motor running but very little air flow coming out of vents. The air flow is always fine when I first start the car, but diminishes after driving for awhile. Mechanic can not find anything wrong since he can never duplicate the problem in the shop. Doing a bunch of google research it sounds like a bad evaporator sensor. Has anyone had to replace this, and is it a DIY job? Thanks!