low revs then stall

had a funny problem last night, it did happen a while ago then seemed to disappear. Started up ok drove off and it switched to gas ok after a few seconds ( its setup to switch at 2000rpm) got about a mile down the road and noticed that when i backed off the gas, going down a hill or something the rpm were dropping very low to about 2-300. done this a couple of times then when slowing down in traffic it cut out all together. cranked it a few times with no joy, swithced back to petrol cranked it again for a bit and it started up. OK i thought lpg playing up its done it before. the couple of hundred yards further on it did exactly the same on petrol, stalled, cranked it a couple of times and it restarted. By now i had got to where i was going. came out switched back to lpg to give it a try and it fired up and i got home without any problems. Any ideas? if its doing it on both lpg and petrol it must be something thats used by both systems so filters etc must be out, cant be the vapouriser not being hot so i really need some help for clues on where to start looking.

Could be worth checking for an air leak on the inlet tract, I've seen all sorts of bits work loose on LPG conversions, engine runs ok at higher revs as there is plenty of fuel around, at low speeds you run short on fuel with too much air via the leak. Look for an air leak and check everything to do with the inlet system is screwed down tight.

Thats a distinct possibility Rob i will check it out when i get time. the fact that it comes and goes would suggest it could be a hose split which only opens up sometimes.

When I first had mine it would idle on petrol fine but on gas it would stop at about 3-400 rpm and stall, so at lights, roundabouts etc.
It would drive fine but not idle.
New Lambda sensor and all is well.

another possibilty Dk although its the same on gas and petrol. Did it again today, seems to be worse when its cold.