LPG Gas, YET AGAIN

I have LPG on my Patrol, any air leak in your manifold system after the LPG mixer will stuff your LPG and make the starts hard. as it will be out of tune. Try fixing the air intake permanently as the metering may be based on the pressure differential across the LPG mixer.

My 4.2 petrol was a little harder to start on LPG now it has got colder in the mornings but it is due for a tune. I like to do the start on petrol so got on to PEEL who did the electronics for my LPG fit and fitted a CP66 Petrol start assist. Starts on petrol when LPG is selected for 2~4 sec worked out fine.
Jugs

it sounds like you have a problem with the gas converter
some converters have an auto prime solenoid, when you turn the key it gives it an extra hit of gas so it starts easier. this could explain why when you switch from petrol to gas with the engine running it keeps going
gas relies on vacuum to draw it into the engine, when they tune gas it is tuned with all hoses and filters connected changing any of these will affect the fuel consumption as will a dirty air cleaner
My advise fix the hose give your air cleaner element a clean with air, blow the air from the inside out not the outside in see if that fixes the problem if not take it to a gas fitter it should only take him 15 minutes tops to find the problem (not to fix it) if any longer find a new fitter

Auto gas systems often incorporate a vacuum gas lock. It requires a manifold vacuum (IE you motor is turning over) to open the lock and supply gas to the engine. If you have a loose hose to your gas mixer you may be loosing to much vacuum at idle/start and the lock is shuting the gas off.

LPG often leaves a sticky oil residue in the heat exchanger / demand diaphragm area. Look for a drain plug under the heat exchanger. Some are large, others are small set screw type. Remove plug and you may get a teaspoon of the smelly oil residue.

Also change spark plugs ! Engine needs better spark on LPG then petrol. Had both these problems on friends cruiser last week, goes like a rocket on both fuels now.

Hey Mate!!
LPG should only ever be used for heating and cooking, under no circumstance should you ever try to run a car on gas.