Here's my latest attempts to solve this problem.
I disconnected the millivolt generator and instead connected in a .75 voltage supply (to make sure the thermopile wasn't the problem). After a few minutes, it kicked off again so I know the thermopile isn't the problem.
I also shorted the connections at the gas valve to take the high temp. cutoffs and wall switch out of the picture, and the problem still occurred.
I checked the pilot flame closely, and it stays in complete contact with the thermopile even when the burners all light.
So, I think I'm down to suspecting the gas valve. Unless there is some other sensor that can shut everything down. Is there more than one thermopile on these fireplaces?
I don't think it is a venting problem, because the flame doesn't flicker at all prior to it clicking off. If venting were the issue, then I would expect to see the fire dance around strangely before it died.
Does anyone know where I can get a detailed spec of the gas valve - it is a RobertShaw 7000MVRLC. When I go to the RobertShaw website, I don't see any specs there.