I have a Bryant Induced-Combustion Gas Furnace, model 373LAV Downflow & 383HAV Upflow. Mounted horizontally. It turns off flame before set temperature is reached, and recycles immediately.

Sequence is:

Inducer starts
After 25 sec. the ignitor glows
After 8 sec. the flame lights
After 45 sec. the blower fan starts
After 2 min. 21 sec. the flame goes out
Indicator light flashes 33
After 5 sec. the inducer fan stops
After 3 min. 58 sec. the blower motor stops
Inducer starts immediately
And it's the same cycle over again.

Clean filter, and no intake restrictions.
Doesn't appear to be cracked heat exchanger (no blowback)
Flames are blue and appear to be confined to the exchanger ports (no roll-out)
Thermostat anticipator has not affect
RobertShaw mercury switch thermostat level

I cannot inspect the vents close-up, they are in high ceiling. But, I cannot detect much air from the vents.

It is 20 degrees F outside. The thermostat is set to 72, but the temperature does not get above 64.

What do you think?

You show a model for upflow and downflow but have it mounted horizontally? Is it rated for horizontal applications? Some are and some aren't. Heat exchangers may be laid out differently. Could be overheating and the thermal overload is shutting down the gas. From your description of operation, it sounds like it is operating properly. I also noted your "low airflow" comment. If you have a collapsed or restricted duct, that will also trip the thermal overload due to an overheated plenum.