Welcome to 2CarPros.
There is a component called a mode door actuator. It is responsible for the air flow direction. Now, since it does work after some time, I suspect it is either going bad or the mode door is sticking.
Here is the bad part for servicing this component. Step 1 is remove instrument panel. I attached a picture of the component so you can see what it looks like. If you want to try replacing or inspecting it, let me know. If you look at the second pic, you will see how the heater and AC unit are behind the glove box. The actuator is on the left side of that unit. You may be able to access it if you remove the glove box.
Let me know if I can help.
Joe
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Friday, September 20th, 2019 AT 7:53 PM