Regarding the stall in temperature rise: Are you sure the burners are properly sized for your kiln?
In my first painful experience with venturi burners, the burners were too small for the 40 cubic ft kiln. We were able to get to temperature after long grueling struggles by people with significant gas firing experience. Some firings took almost twice as long as they should have. Results were interesting. We spent most of a semester doing this before the size issue was identified. Smaller burners were replaced with larger ones, and VOILA the problem was solved.
Now, that kiln was being fired with industrial pressure piped natural gas. But, for years before, the same kiln was successfully fired with two tall bottles of liquid propane and forced air burners in all types of weather conditions including freezing during the winter.
Just a thought.