Idle make your mind up??!!

An odd one this, my 88 classic has an idle issue, when you start it sometimes it immediately ticksover around the 800- 900 mark, happy days, however every time i come to a stop during my journey the tickover seems to be somewhere else, so low sometimes you cant decifer it on the tacho, and also if your a bit quick to select drive or reverse you can stall the engine. It gets worse the hotter the engine gets. I should add my engine only runs on lpg (read: no fuel system) and I believe that to be fine. Could this be an electrical gremlin dizzy, rotor arm etc or could my vac hoses be shot or something else like a component breaking down when hot? Help please!!