Have all work permitted and inspected. WE KEEP GOING UNTIL YOU HAVE THE INFORMATION YOU NEED. I come and go. be patient.
It is likely that the draft created by the main burner is pulling the pilot flame away from its flame sensor enough that it fails to sense flame...and shuts the gas valve off.
you can use compressed air, or one of those cans of compressed air used to blow dust off of computer electronics to blow off the pilot light from underneath it, thoroughly, using the long plastic tube that comes with the can.
then use plumbers sand cloth to clean off the end of the flame sensor / thermocouple device.
See how that works.
Observe the pilot flame behavior before and after the main gas valve lights.
Let me know, we keep going until this resolves.
Phil-Money well spent! got home and did just what you said and bingo...what do you know a working fireplace. I even shut it off 3 times to make sure it was going to keep working...Too think one of those repair places wanted $150 just to come to the house and over the phone they sounded like they were already to charge me for a "valve" for another $200. I'm sure they new just waht was going on just like you did!