I have an old vermont castings wonderfire direct vent gas stove. The pilot fires and stays lit, the stove will fire and run for a minute or so, then something shuts off the gas valve completely. The stove and pilot both go out. After another minute or so I can then relight the pilot and the stove will refire, but it will again shut down after a minute or so. Does anyone have any ideas on troubleshooting the thermopile and or gas valve?