Never heard of "Josten" brand fireplaces and a quick Google failed to reveal them. Is that some local store you bought it from? Try to locate the rating plate with make and model.
Flame should not be indigo blue on a direct vent--should be blue bases with nice prominent yellow tips. I hear what you're saying about thinking the venting is perfect but what you're describing could be ghosting out or oxygen starvation. Try to watch the fire as its about to drop out. If it turns a transparent blue and the flames lift off the burner flying around inside like ghosts, then you have a venting problem. Sometimes its just a failure to remove all the insulation packing from the vent collar on the unit itself, sometimes a damages section and sometimes on long runs, it becomes too much as the pipe heats up and the thermosyphon effect fails. That's a high closed mv even though jumpered out. Try to get burning mv TH-Tp/TH. Feel the valve just before its due to drop out. Do you think the valve is getting hotter than 175F, which its rated for? Look at the pilot flame after burning for awhile. Does a previously stable flame begin to wobble and pull off the TC? Again, that's usually a venting issue. No mention of inlet gas pressure under full load with Fp and all other gas appliances firing.
If that doesn't work try Guinness.
HTH,