I have an Olsen propane furnace. Periodically the forced-air blower will stick "on" and continue blowing as the house gets colder and colder. While the blower is on, the buners can't re-ignite. The flashing red light on the circuit board indicates that the system is locked down. If I shut off power to the furnace and restart it, it will run fine for a while again. I've had a new venter motor, new circuit board, new flame sensor, new high limit switch and new wall thermostat installed, but the problem with the blower staying on persists. What might the source of the problem be?