There are a few possible reasons: In the summer, warm moist air leaves moisture on the heater, then drops extinguish the flame at the base by temporarily covering the orifice. Dirt has fallen, partially blocking the pilot flame. Opening and closing doors, starting clothes dryers or fans or windy conditions change the draft in the chimney and the pilot wavers away from the thermocouple sensor just enough to drop out the pilot solenoid valve. The thermocouple is aging and is on the borderline of being able to keep the thermocouple solenoid valve open. The wiring in the circuit is not absolutely 100% perfect, making a weak current. The gas pressure or the quality of the gas mixture is periodically off optimum. It could be one or several of these causes. I'm sure it is not Casper the Ghost - but you never know.