Heat-n-glo fireplace, fire goes out after a while (after 20 min or sometimes up to 1 hour) but pilot stays on. Problem started after Gas to the house had been turned OFF for the day. That was a month ago. Often it's not even fully warmed up (eg. 15-20 minutes) and other times an hour or two. So guessing it's not a heat sensor. Any air in the line should be looooong gone. Replaced thermopile 13 months ago. How can I eliminate the most likely possibilities?