auto climate control duct actuator

I have a '02 Yukon/Tahoe with automatic climate control. The actuator that controls whether the air blows out of the defrost vents, the a/c vents or the heater vents, oscillates. What happens is the air comes out of the heat vent for a few seconds, then the defrost for a few seconds, then the ac vents for a few seconds and keeps repeating. Is there a position feedback internal to the actuator/motor? Is there something else that gives the command to stop moving the actuator? Is it time to replace the actuator? Is it something that needs rest in the computer? etc?

Thanks in advance.....

I replaced the mode door actuator and it did the trick. Works like a champ.