Main burner in NG fireplace goes out
Make sure that any operating device and the thermostat are made to be used with a thermopile/powerpile system. The contacts of all devices are special for that type system. Thermopile systems produce around 3/4 volts so any added resistance reduces the voltage to operate the gas valve. Make sure that all connections are done with clean wire and are tight. When the system was converted did the person or contractor convert/replace the pilot, gas burners or gas orfices and the gas regulator. It may be a good idea to check these items or have an HVAC contractor check them.