I have had this problem for a while now. the car idles fine on petrol but f@rt about on gas. it drives fine, just doesn't idle well. I have tried all manner of things to try to solve this problem.
so far i have:
new leads
new coil pack
new plugs
gas service

I also had a problem with it driving badly on petrol, so i changed the injectors over with some au ones i had AND NOW IT IDLES PERFECTLY ON GAS AND PETROL.
i can only assume that the old injectors must have been leaking some fuel in when it was on gas, making it idle bad.

Hopefully this might help someone else.

I had a similar issue with my EF...not sure if it relates to you, but my Mechanic found that some fuel regulator at the back of the engine (Has a diaphram in it) was torn and was allowing fuel to flow in while on LPG and giving it a rough idle....once he swapped that out things where fine. Also could be your MAP sensor.

Just throwing tips your way from my past experiences with an EF i used to own