Running too cold on LPG

[Mallard]

I picked up an AUIII Futura duel fuel a couple of weeks ago and have noticed an issue with the LPG.
Most of the time it runs pretty sweet but on the trip home from work it develops what feels like a miss.

To make some sense of it, I drive from Wollongong to Sydney every day and on the trip home, going down Mount Ousley is a 9k roll down the hill where I don't use the throttle at all. I drop it in to 3rd (auto) and coast. Even then I need to brake occasionally to keep it around the 80k limit.

From what I've read LPG cars need warm coolant to help vaporise the gas. I expect by the bottom of the hill, the thermostat is firmly closed and the water in the system (apart from what is in the engine) is relatively cold. The heater certainly has lost a lot of heat.

In normal driving the thermostat sits between the N&O (Ford fitted the new one just after I got it). I replaced the plugs with NKG IGR5C13 plugs as that was what the NKG site recommended for both petrol and gas. Leads to be changed this weekend. I want to put a larger CAI and K&N filter in but am worried it might aggravate the problem.

I expect this problem to get worse as the weather gets colder unless I can find a solution.

Any help greatly appreciated.