Vectra SRI LPG Coolant Problem!

Hi there, can anybody give me a bit of advice? I have a Prins VSI system
fitted to my Vauxhall Vectra and i have been having a lot of problems
recently. It started about 2 months ago, i could run on gas but only for
about 10 mins then it would switch back over to petrol on its own giving
me a loud beeping sound from the switch. I thought this might be a
coolant problem because the reducer never felt that hot and i thought
this might be why it was switching back over to petrol. I flushed out
the system and replaced the coolant thinking this might solve the
problem. Now i have noticed that when it it running on petrol,
everything runs smooth and the coolant level stays pretty constant, with
the bonnet up, i switch it to LPG, give it 10 seconds, the coolant level
rises rapidly in the expansion tank and starts to spurt out everywhere
even when the lid is screwed on! I have run the engine with the cap off
the expansion tank and the heater on full to get rid of any air locks in
the system but this hasn`t made any difference at all. Do i need a new
Vaporiser? If so, what kind of price and does it have to be prins or
will another fit my system?
Sorry for the long essay but i wanted to get as much detail in as poss
in the hope i will find a solution quick, costing me a fortune running
on petrol!!

Thanks in advance, John