Hi,
The "coughing"/unstable flame later in the run problem is heat. If you look at the right water tank where the gas tank is located, you can see the problem.
The gas tank is totally enclosed next to the boiler and smokebox. Look at your other locomotives, the gas tank can get very warm, but it usually is at the back of the locomotive. It usually has some exposed surfaces to the air. This one has none AND the gas tank goes to the front of the tank right next to the smoke box, the hottest part of our locomotives.
I will stuff some insulation between the smokebox and the right water tank/inside gas tank. Hopefully, this will stop the gas from overheating.
Lots of people are speculating about the problem, but this is not one of the "round up the usual suspects" problems we have with gas fired engines.
I will let you know with the insulation experiment.
Dan