hmmm, fan belt, I'm not sure his unit is that old. I haven't seen a
fan belt on a residential furnace ever.

I agree with the low pressure of the gas, the regulator maybe.

Another thing that was eluded to is this, the thing that starts your
fan in heat mode is a temperature sensor in the furnace. When the gas
first ignites, it heats up the heat exchanger and a temp sensor senses
that the furnace has warmed up and then turns on the fan. Also, when
the gas shuts off, the fan continues to run until the temperature
sensor in the furnace gets down to the shutoff point.

How long do your burners shoot out flames before they shut off? If
it's more then about 15 seconds, then the furnace temp sensor that
tells the fan to come on may have not been working and the burners
stayed on until the high limit temp sensor cut them out, the furnace
cooled some and the burners came back on but then the high limit cut
them out again. I'm guessing maybe the temp sensor that turns the fan
on was flakey or if your unit is new enough, the electronic board
malfunctioned and the reset cleared it.

The high limit temp sensor is a safety and keep the burners from
running too long without the fan blowing, ie, the furnace would get so
hot it would burn up the house.