I have a Majestic / Vermont Castings DV580 rear vented - with a strange problem. When the fireplace is off the pilot light seems to run fine. I have no problem starting the fireplace, but it will run for a variable amount of time (usually 15-20 mins or so) and then suddenly shut off - the main burner and pilot light extinguish and a few seconds later I can hear the gas valve close with a "click". It then seems like I have to wait about 15 minutes until I can restart the pilot light - if I try sooner the igniter just sparks but the pilot won't light. Can thermopiles get too hot and not produce voltage? I find I can run the fireplace 10 minutes with no problems, but if I push 20 minutes it will almost always shut down. I have had many "technicians" come out and they are stumped. They have replaced the thermopiles (twice), removed the shield around the pilot (and replaced it), replaced the gas valve, put fibreglass insulation around the gas valve, cleaned the burner, repositioned the logs many times..... Of course the dealer claims no responsibility because the contractor who built my house installed the fireplace. I am at wits end! Our winters are cold! Any help would be much appreciated!!