I have used a ventless natural gas fireplace for the past 5 years in our living room. We understand the concerns about these types of units and have taken precautions to protect our health and home. Our 1900 house breaths better than I do!
Last year we had an intermittent problem with the fireplace shutting off during use with an audible 'click'.
This year the issue has increased and the fireplace will only run for a few minutes before shutting down, pilot included.
The pilot re-lights immediately and the burner can be re started but again after a few minutes, click, everything goes out.
I purchased and installed a new thermocouple thinking it was going bad but alas, have the same problem.
It is almost like a safety feature for over temp that seems to be controlling this.