It could be down to either a leaking injector or leaking vapouriser filling the manifold with gas - if you can find the vacuum line to the vapouriser pull it off, plug the end that goes to the engine to keep it running and see if you have gas leaking from the vapouriser vacuum connection (it may be more visible by putting a piece of the hose into water with the other end connected to the vapouriser then looking for bubbles if its a small leak). Alternatively you may smell lpg when you've stopped the engine, if this is the case its leaking from somewhere, and the symptoms would indicate its filling the manifold. You may be able to detect in the same fashion with the injector hoses if you can get to them (or if you can monitor pressure, if it drops slowly over time once switched to petrol this would indicate a leak).http://lpg-kits.com/
lists a compatible cable, Tinley tech are now the UK BRC distributor so may be able to assist with a cable? You should be able to do most of the above without the cable though, and its unlikely the cable will help just yet though they can be useful to have.