LPG idle misfire issues

[Jason t5]

Hi all I'm looking for some advice on my v70 T5 2003. I've had the car 6mths now and the gas has been a nightmare (it was fine when I bought it) I had new injectors fitted as one went down.. all was well for a while but then the car develops a rough idle and misfire on LPG when cold. When my LPG installer remaps the gas all is well for a couple of hundred miles and the problem keeps coming back.. it starts off with a splutter on changeover and progressively gets worse to the point where it's running rough on LPG until I've done about 30 mins of driving and then it seems to sort itself out! The odd thing is if I keep the revs over 2000rpm it's fine and if I switch to gas say on the m/way it acts normal? Every thing is running fine if I leave it on petrol! I've also had the ECU changed on the LPG system but it's still acting up.. any advice on what else to look for please this really has left me and my LPG tech scratching our heads. Many thanks