What's the problem when the HVAC airflow becomes restricted after 30 minutes? on 2001 Chevrolet Tahoe

Whether I select AC or heat the airflow becomes restricted to 25% of original flow after 30 minutes of operation and then goes down to about zero from there. The blower motor is still running at the same speed set. Checked other vents to see if the flow is being directed somewhere else, it is not.

Asked by for the 2001 Chevrolet Tahoe
ck cabin air filter if applicable on your vehicle
Checked filters and they were dirty. Changed filters but still have the same problem.
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