The pilot light on my water heater, FGR 40-224, will light manually, stay on, and then burner will then light. But, once the the water heats, the gas turns off and the pilot light goes out. After tank water cools I can do the process again. Replaced the thermocoupler and problem persists... Should I replace the thermostat gas valve next. Trying to keep the tank water going until the tankless unit is installed.