sounds like the petrol ecu is adjusting whilst on gas, this it will do its normal, the problem is when the gas and petrol trims are out of sync.
for example (and this is only to explain a situation, not actual data)
if the cars running rich on gas and lean on petrol there will be quite a difference in the lambda readings, the car will adjust the petrol trims to suit the gas, i.e lean it out. this will have no effect on the lambda reading so then it will try some more until it is way out!
you need to get the gas set up correctly. deteriation of air filter can make a huge difference on single point systems, i would imagine it will make a difference on multi's but prolly not as much. also any of the ignition system thats a bit suspect is worth sorting first
if the car is due a service do that first, run it on petrol for a few miles and the trims will drop to around 12% i think.
now you need to set up the fuelling on the gas. this is done in many different ways depending on the system but on multi's you need to press whatever button calibrates it in the software
you can test the theory by switching over to petrol whilst warm see if it runs crap or picks up after a short while.
incidently if your car has the correct 50/50 antifreeze water mix you can run most of the time on gas, even start from gas. the way to do it is (there a few different methods but these two are the most common i think) if you have a switch in the dash for the lpg, flick it to gas then petrol a few times ending on gas, start car.
if its a button, with ign onpress and hold button for 5 secs then proceed to crank engine. in both cases the lpg light should be solid!
oh and after much thought i suppose i am rough and idle!
