2001 AUII factory dedicated Lpg falcon ute missing

[scooto57]

hi i have a 2001 AUII factory dedicated lpg falcon ute and it seems to be missing.
i tried taking the first lead off while it was running and it makes no difference same with the 4th cylinder. the plugs are only 10000km old and they are the $24
ones that are supposed to last 100,000km. i tried replacing the 1st and 4th spark plug but it had made no difference. the leads seem to be arching out. but they are only 10,000km old aswell i have tried some extra insulation around the ends with electrical tape to see if that made a difference to the arching out. but this has done nothing.
just wondering if someone has a solution to this or has had it before.
thanks