Been playing with this AGAIN
So a friend of mine has done some LPG installations and I've ran it past him a few times. Also a qualified electrician so we've checked quite a bit of stuff. There was voltage difference that we've evened up, though it made little difference and a few pipes we've straightened and then on a run out with the car he noticed it was over fueling that side of the engine on LPG and there were other misfires and stuff happening.
It led us to manifold pressure and the vaporiser. I had a "spare" from my old S8 and a Prins kit that i removed various bits off the old car before it getting passed on.
It looks like to get a manifold pressure feed for the vaporiser they put a port in the opposite side of the manifold to cylinder 8. We suspected a diaphragm leak on the kit, which then feeds excess LPG into the manifold and causes problems. It would certainly explain the over fueling and why No.8
The hot water hose had a kink in it meaning it probably didn't get hot enough and damaged the vaporiser!
With a bit of a fiddle we swapped them and now back to trying to tune things, but we don't seem to be getting No.8 cutting out