I have a two zone heating system (Lennox elite series) furnace. Zone 1 works fine. Zone 2 does not. Thermostat call for heat. Burners ignite, but doesn't stay lit. After three minutes of cycling on/off the system goes into the safety watch guard mode(alternating slow flash). I checked the dampers they are working fine. Once and a while zone 2 will fire up and work fine??? What could be the problem considering zone 1 works fine? Bad thermostat?