P38 LPG problem

I have a 1998 R reg P38 4.0SE Auto with a Landi Renzo Ecogas LPG conversion, the problem with it is ,it will not turn over to gas when the engine is cold, you can drive for miles and it will try to change to gas but seems unable to do it. Once the car is warmed up, stop the engine stand for a few seconds, restart, and it changes to gas almost instantly.
When the car refuses to change over the indicator in the car appears to tell you you are out of gas, I have had it checked by a local firm who replaced an earth wire which they claimed was faulty but is hasn't cleared the problem, any ideas?

A trick which works on mine (if your display is the same) is when the light flashes to say it's ready to change over, get the revs over 2000 then let then drop down to about 1500. If your on a motorway dip the cluth or stick it in neutral, once the revs drop it should switch over to gas. Hope this helps.

Mine doesn't give you any warning it's going to change over, it just changes and fails.