Here is my problem: When I start the car in the morning air blows trough the vents as usually. But after 30 minutes of driving airflow decreases until nearly no airflow trough any of the vents is noticeable. It does not make any difference if A/C is running or not or what setting on the HVAC-control I use. I also can hear the blower motor in every switch-position, and the hissing noise when moving the HVAC-****.
When I let the car cool down completely, everything works again.
What could it be? I have searched the board, but i found no one with the exactly same problem. Thanks for any help,

Daniel