Hi! I had the missfire problem for over a year. On the petrol only. Its an y reg convoy. I've known that there had been a problem with the catalyctic converter because of the noise but it ran smooth on lpg. Once I've replaced the converter it worked fine both on petrol and lpg. I think the petrol system was clogging up within the catalyctic converter and was sending a strange message to the engine. Sorry, don't know what to say about your problem. Just wanted to share my experience. I think the lpg system on mine is not the fctory made one, but according to the receipts got when I bought it, it was installed by West Yorkshire Autogass.
cheers, good luck
Vlad
April 2009