My pilot light keeps going out on my gas water heater. I re-light it just as described in the instructions - click the spark device several times while holding down the button, continue to hold down button for a full minute after flame appears, turn the gas knob back to ON, set the thermostat. And the light flames up big and stays lit for 10 to 20 minutes or so while it is heating the water to the level set on thermostat. But when it reaches that temp the pilot light goes all the way out (instead of just going back down to a small but still lit flame). I have repeated this process several times now with exact same results. Any ideas ? Or must i call a professional ?