My gas furnace has developed this nasty habit:
Some mornings we wake up to find no heat, but the furnace fan running
continuously.
Even when I turn off the furnace at the thermostat, the fan continues
to run.
If I cut power to the furnace (I usually just pull the door off), and
then back on,
the furnace will fire up and run normally. This has happened about
four times this winter.

Any ideas?

thanks!