why does my gas fireplace shut off

I don't think it is a thermocouple problem, or water in the gas line as I have seen this here in California which sounds similar. The fireplace turns on with a nice flame and then goes to blue and then woof, it and the pilot flame are extinguished. This only occurs in the minutes after start-up and does not occur when the fireplace has been running for more then ten minutes. Cause: At turn-on, there is significant oxygen and the fire consumes the oxygen, but there is a problem, the consumed gas needs to push the heavy and dense cold air in the chimney up out of the pipe in order to draw fresh oxygenated air down the biconical pipe to feed the fire.

During that critical time, if the exhaust gas is not able to push through the cold air in the exhaust stack, no new air is drawn in and the fire quickly runs out of oxygen and the flame and pilot light are snuffed out. This gets worse on cool days. To solve, make sure the exhaust stack and intake pipes are clear. If that doesn't work, you can try this, you won't like it, but it works for me: you watch the flame at start-up and turn it off before it turns blue and extinguishes itself, then wait two minutes, try again, but be sure and turn it off before it goes completely blue.

On the third attempt, there should be enough latent heat to force a channel of air up through the dense cold air and then your fire should continue burning. I plan to look at side venting to avoid the cold exhaust stack problem.
I've had a gas fireplace for about 5 years in my master bedroom and it has worked fine. until recently.

The problem is that when I turn the fireplace on (via a switch on the wall), the fireplace fires up and all is good. Then after about 4 minutes both the fireplace flame and the pilot light kick off. I relight the pilot light and the same thing happens after a few minutes of turning the flame on. I had a service man come out and he fiddled with it for a couple of hours. He replaced the millivolt generator (G01A-332), but the problem still occurs.

He seemed perplexed. He offerred to replace the gas valve but said it would cost $XXX. Does anyone have any ideas on what I should check. I've checked all the wires and don't think the connections are bad. I measured the voltage being generated by the millivolt generator. With just the pilot flame, it reads 555mv. When the main burner kicks on, it drops to 279mv. Is it supposed to drop down when the main burner kicks on?

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