A few thoughts:
The petrol system must run properly for the LPG system to get the correct signals to convert to LPG injection signals. Providing the wiring of the LPG system isn't faulty, ie the injector cut wiring is fully intact, then you need to concentrate on the petrol first.
Now if its only running on 4 cylinders, one of the coil packs may be unplugged. If the LPG computer takes its RPM signal off one coil (like I do it) and that plug wasn't fitted properly, it could jump off. I had an alternator regulator plug jump off my wife's car a few weeks ago, that was nearly embarrassing. So it's easy enough to do.
Without any other knowledge, I'd also be looking at a MAF (mass air flow meter) failure, especially if the mixtures are really lean or rich. LPG won't fire at levels that petrol will just manage.

