The pilot light was out and we don't have any hot water. I re-light the pilot and the heater starts burning, however, after 10 minutes, it stops burning and the pilot goes out as well. Once it goes out, I have to wait a few hours for the whole unit to cool down before I can re-light the pilot, then 10 minutes later, it shuts down again. Any idea what is wrong and what I need to do?