Fireplaces, Heating Stoves, Flues and Chimneys - Gas Fireplace keeps going out

I recently had my gas fireplace serviced because it would not lite correctly (apparently the pilot assembly was not even bolted in correctly)

Anyhow, now the fireplace was coming on within a few seconds and working great. but now i have a new problem...

After the fireplace is on for anywhere from an hour to 4 hours, the fire place will make a pretty loud 'CLICK' and shut off, pilot and all.

If i re-lite the pilot and turn it back on, it will click again and go off within a few minutes.

Any idea's? Before i had it serviced, i could keep it on for many hours with no issues, just had issues with the inital lighting.

Thanks

John

p.s. This is a wall mount, vented fireplace, no fan and not sure on the mfg. the service tech said there was good gas pressure and he cleaned everything up.

Will it relight and stay lit, once the fireplace cools back down? If so, it sounds like a thermostat simply shutting off the fireplace. Or, it could be a spill switch (that senses the temperature at the flue) that finally got too hot. Or a bad connection at where you might have a thermocouple and spill switch that come together by the gas valve. If these parts, that screw into the gas valve, come loose/corrode a little, it will lose contact. Heat expansion could be the cause of lost contact. Just a thought.