Or it's just a crap multi valve...? Ime they do trip out to Petrol at the slightest jolt. Cats eyes, bumps, corners even.
The original multivalve work perfectly, until it stopped completely. I never had any of these problems with it.
We might be onto something here. 
False trips occur when the vapour pressure falls too low for the engine to continue running on LPG.
Give that the multivalve is what caused this problem it would make me wonder if it's not capable of flowing enough liquid to feed the engine. If this is the case, it might well be causing the engine to run lean, and hence trim the fuel to the point where starting becomes awkward.
That's what I'd wondered.
Gauge readings could well be because a different multivalve has been fitted without the display being calibrated to suit it.
Yes, I'd reached the same conclusion.
Issues if the fuel is consumed fast make me wonder if the pickup has fallen off the multivalve. In this case, the valve will be delivering vapour instead of liquid. This restricts the flow of fuel causing a loss of pressure (see my first paragraph!) and, if the gas is being consumed quickly, the temperature of the tank can fall so low that the gas can't boil off vapour any more, so the system behaves as if the tank is empty while the liquid level (as shown by the gauge) remains high.
The only thing that changes with motorway driving is the gauge reading. LPG performance is the same.
I don't think the pickup has fallen off, only because the behaviour hasn't changed since they "changed" it. They took the tank out of the car and re-fitted it a few days later. I suspect they took the multivalve out, couldn't see anything wrong with it and re-fitted the same one, but I do have my suspicions about the multivalve. The BRC chap in Italy muttered something about "might have to find a secondhand/reconditioned unit" when I first spoke to him. (it's not the usual size/type they use down there.)
If you take it for a decent run (say a couple of junctions up the motorway at a decent cruising speed) then get out of the car and feel the tank, is it cold, and perhaps showing signs of condensation or frost on the outside? (it should stay at the same temperature as its' surroundings).
It would also be worth checking the temperature of the vapouriser at this point, to make sure that it is piping hot.
Chance would be a fine thing, although I have done the vaporiser test and it was very hot to the touch.
) , I've been running on petrol for a few days now and have not had any problems; no misfiring, no hard starting; although the eml has finally come on again (pre-cat issue surely).