96 Tahoe heater blower

Hi, the heater runs fine for about a 1/2 hour then all of a sudden it stops and it is like something is keeping the heat and air from blowing through the ducts and dash vents. If you stop for a while it will work fine. Thank you for any help!

Automatic AC controls?
Does blower motor quit running or other problem?
Does switching blower speeds or moving control levers help?