Like classic swede says look at the solenoids. These run hot and in time can crack their casings, although a crack in one doesn't mean to say it's failed. The inline one is on the side of the vapouriser.
Below is a picture of two Koltec vaporisers showing their solenoid cracks. One is a Volvo one and one was a Vauxhall one i think, but same part.
http://www.volvoforums.org.uk/pictur...pictureid=6368
If it is a Necam/Koltec system then these run fine but many don't know how to properly test them and replace parts willy nilly.
What you really want is to measure the gas pressures. If you're not happy doing this get the volvo dealer to do it for you. On Necam distributors there's a little side port for this. The pressure there should be stable at about 14psi above atmosphere. If it's not go backwards through the system and measure the vapouriser pressure on its side port. That should be stable at about 21 psi above atmosphere. You'll see it shift a little with demand but not much. Also it may overshoot a little when the LPG is turned off.
If the car is stalling or not running at all on LPG then I'd look at the pressure at the vapouriser. As soon as the inline solenoid opens this should immediately jump to about 21 psi.
It'll become more obvious where the fault lies when the car dies on LPG when you look at the pressures.
Some LPG garages will replace the distributor when they don't know what they're doing or cant be rrss tracing the fault. Unless there's some underlying fault Necam distributors don't expire after 91K miles. One by me wanted £500 for a new one! I completely overhauled it with a seals kit for about £15 inc post.