[SnineT]

Tom if you're still reading this and haven't sorted it yet consider this, about 2 decades ago when LPG was the new Diesel quite a few people I knew at the time rushed out and had aftermarket kits fitted, some of the from factory cars are still only add on kits believe it or not, anyway most reported back the LPG was 50% of the price of petrol but used twice as much, one good friend had a 2.0 Omega and that eventually started playing up with a misfire which in the end he took the easy way out and got rid of because nobody could tell him exactly what was causing it, I suspect it was probably a pump somewhere failing but we'll never know and he's no longer with us,

The diagnostic tools will struggle because they're expecting a petrol or a diesel engine, when you add the LPG in you're asking some components to do two jobs and others to sit and wait their turn,

For some easy checks if you can see where the LPG is injected check for any leaks, does the pump nearest the LPG tank sound ok or is it louder than normal, you also need to consider it might be the petrol side of things at fault as well, a lazy pump might cause a disruption in switch over supplies, one thing I would recommend is you do a full plugs and filters service on it to narrow things down, a worn plug will misfire, a bad lead so check you're getting a good spark or dirty fuel and air filters causing the ECU to compensate, a bottle of fuel treatment will help as well.