I have a direct vent gas fireplace with a standing pilot light. The fireplace has not worked for me since I recently moved into the house (15 yr old home.) The gas line is open and I have a nice good blue flame on the pilot but as soon as I switch to the "ON" position the light burns out immediately. I have tried purging the line for minutes at a time. The pilot itself is clean. I have replaced the 820 SIT gas valve. The pilot thermostat has been adjusted several times. I have replaced the thermocouple. No luck thus far. Stumped at this point. Any suggestions are welcomed and greatly appreciated. Thanks!