AC/heat fan door closing

Yes Ive been having a problem with my vehicles ac/heating. Whenever your using either one of them youll be driving and it will quit coming out of the vents. The fan is still running you can still feel the ac/heat which ever one im using at the vents. Sometimes you can drive an hour or so before it does it. Ive stopped before got gas or something and start it up and its working fine for a little while longer. I really don't know where to start. Hate to just start replacing parts. I thought about just rigging the door open but hate to do that as well. Well hope somebody out there has seen this before. Thanks.