Then it could be that the rate of evaporation is being reduced by the reduction in temperature to the point where it is having an effect on the flame. Some modellers warm the tank with a line from the engine exhaust running under the tank but this can then cause a lot of spitting from the funnel as you condense some of the exhaust steam.
Possibly the easist way is to simply be aware of it and, if you have an "auxiliary tank", simply change them over as they become cool. The stand by tank should then warm up again and return to a normal pressure. It is also well worth remembering that the rate of cooling is proportional to the rate of evaporation in the first place so being as careful with your gas consumption place plays a big part. If you can possibly initially raise steam pressure on the bank from yet another tank that will be really beneficial to maintaining the temperature in your gas tank, which will then not even start to fall until you put the model in the water.