misfire on deceleration

I have a 1996 range rover 4.0 auto SE running on petrol and LPG. I have a problem on LPG when i take my foot of the accelerator it misfires/backfires .It does not do this on petrol and only at higher speeds on LPG.I have changed the plugs and put a set of magnecor leads in.I have also had it on ramps to see if there was any holes in the exhaust but that was ok.Any one else had this problem or any ideas as what could be causing this i would be gratefull for your comments.

are you sure its a missfire and not just popping due to it fueling on over-run?

Yes it is more of a popping.How do you cure this?

Just got it connected up to see what faults were showing .Turns out two of the lambda sensors need changing.so ordered two hope this solves the problem.