When I firmly brake in my EL Futura, the revs will go right down to around 250rpm , and sometimes it will stall unless I save it by hitting the accelerator a little. It idles perfect on both LPG and Petrol, yet the stalling when braking is much worse on LPG, though an intermittent problem, to varying degrees.

I've replaced the Inhibitor Switch on the Transmission and the Distributor along with it's sensors, new plugs and leads, new battery...

LPG is not factory so it's not ECU related.

Apart from that the car runs fine and I have no idea what it could be, any help much appreciated!

brake booster leaking?