Did you say with LPG? What plugs have you put in? I think the MAF idea may be plausible too. The other thing that is ringing in my mind is whether the timing belt has jumped a tooth. You'd get erratic running but I don't know whether it would register as a misfire.
Also,I wish you'd stop bloody quoting me. I deleted the second post straight after the first as I realised it was a mistake and Tunnie had already flipping quoted me.