I have a Lennox furnace model G23Q3/4-100-2. When the thermostat calls for heat, the igniter will light the pilot, the gas will come on and light, but the blower does not come on and the furnace will shut off after a few minutes (overheats because of no blower). I can turn the thermostat to on instead of auto and the blower will turn on and the furnace will work correctly. I replaced the thermostat and still have the same problem. The furnace is 11 years old and the control unit (Johnson controls G776RGD-14) has been replaced by a technician twice in the past. Is the control unit the problem again? Does this unit have a history of problems?