I watched the unit with the cover off.
It seems to work fine just fine for the first 10 mins.
The furnace turns off after 5 min, presumably when the high limit
switch protects the unit from over heating.
It blows air for about 2 mins, and reignites and heats up and shuts
down.
It seems to operate, but its just that after a while it stops heating
up.
I have the thermostat set at 74F, while as it only gets up to 68F.
It seems to behave as if had reached the desired temp.
Maybe I should check to see if there is still a call for heat by
seeing if there is current across the C-Y leads [call for heat by the
downstairs thermostat.

It's not clear from your description exactly what the furnace is doing.
Normally, when the thermostat is calling for heat, the burners come on and
heat up the heat exchanger, then the blower comes on, and the burners and
the blower both run until the thermostat is satisfied. Then the burners shut
off and the blower runs for a short time until the heat exchanger cools
down, and it turns off. Is this happening?? You say "I have the thermostat
set at 74F, while as it only gets up to 68F."
If this were true then the burners and blower would never shut off (with
normal airflow) because the thermostat would never be satisfied.
Are you sure it's not "cycling on the high limit switch"? You can tell
because after the burner initially comes on and heats up the heat exchanger
and then the blower will come on & stay on. The burners will heat the
exchanger up past the maximum safe temperature, and the limit will open to
shut the burners off, but the blower will stay on to cool down the heat
exchanger (and therefore the limit switch) and then the limit switch will
re-close and allow the burners to come back on. The cycling on & off of the
burners tells you there is a problem. Probably an air flow problem as
somebody else previously mentioned (weak blower motor, slipping blower
wheel, loose duct liner, plugged A/C evaporator coil, etc.) You need to get
a qualified tech to check it out.