Final follow up comment - I promise.
I searched this site on lpg misfire and on this thread:
http://www.volvoforums.org.uk/showthread.php?t=76567
Came across this brilliant entry by POUCH:
Hi i also have a 2003 bi fuel 140.000 on clock, iv'e done 70.000 in last two years,when i bought the car it had a missfire, so i disconected gas pipe below the filter,a small hiss is heard as the gas in the pipe is relessed,no problem just dont smoke
Then i poured half a bottle of STP injector cleaner down the pipe,reconected the pipe,started the engine waited for auto switch over,the car would spplutter and stall,then i leave the car for a few hours to let it desolve the oil deposits.
Then when i start it i takes a few seconds to clear, then runs like a dream,i do this process when ever it starts to miss or stall,or every 10.000 milles.I have tried diff types of cleaner and found this one to be best! good look
What a brilliant, easy and economical answer it has proved to be - my engine management light has now gone out!
I was anticipating planning a Saturday stripping down and cleaning the distributor as others have indicated it may solve the missfire message problem - what a relief.