I have a Lennox gas fireplace that has issues with the pilot light. The pilot light works fine until i turn on the flames and leave it on for 20 min plus (until it gets really hot). Once i turn off the flames the pilot light turns off in a few seconds. If i try to turn it back on it wont stay lit. Only when the fireplace cools down then it stays lit with no problems. I thought the problem was the thermocouple but if it works fine when the fireplace is not very hot after minutes of use i suspect that is not the case. Any ideas?