From what I just read the the LPG uses a secondary mixture computer which "intercepts" the engine management signals and uses those for the LPG system. I wonder if it is possible you have a torn air boot on the MAF to throttle-body connection that is throwing the codes for the petrol powered system but the LPG has more computer controlled adjustment so it can keep running without problems.
The condition you described certainly sounds like how our X was running when the rubber boot slipped off of the MAF (on the side going to the engine).
Hope it helps!
Last edited by Zulu95; 12-05-2012 at 12:38 PM.