How can you go this long without a furnace?
After watching your video again, I am going back to my dirt answer.
What you will notice with not enough return (intake) air is the flames will light normally and run for awhile. In anywhere from a couple to several minutes, the flames will shut off even though the thermostat isn't satisfied. The blower will remain running like it normally does after the flames shut off, except after a few minutes the ignition cycle will start again. It is normal for the blower to stop at the start of each ignition cycle, so that is not an indicator. The only way you can be sure that the thermostat has not reached setpoint when the flames go out is to test across the "R" and the "W" low voltage terminals. If your meter shows no voltage, the thermostat is still calling. If you have voltage from terminal to terminal, the thermostat is satisfied.
There is a limit switch that senses the air temperature downstream of the burners. When the switch gets too hot because of a lack of airflow, it kills the burners and keeps the blower running until it cools off and the cycle starts again.
If this is happening to your furnace, it is a seperate issue from what is happening in your video.
2000 Sierra 2500 HD. 408 (11-2007) Whipplecharged, Big (18x24x4) FMIC.