The 1st time the thermostat calls for heat, furnace runs through a cycle correctly & shuts off. The 2nd time heat is needed, the flame will light and then shut off after a few seconds. Sometimes it will go off during a heating cycle. Often I hear a crackling or clicking sound right before the flame goes out. If I turn it off & come back later, it will usually run correctly for one heating cycle. Cleaned the flame sensor; no change. New flame sensor is good for a few months before this starts again.