Need advice on LPG problem.

Sometimes when driving along @60-@100 kph the van will start to chug, what I have found is the second solenoid before it goes to the converter (this has the filter too) starts to freeze up. the tank could be full, half full, or near empty, makes no difference.

Be it hot, cold, or warm weather, or hot, cold, or warm engine. Makes no difference.
Might do it twice in one day or once a week.

My thoughts are this.
Its trying to pull too much out of the tank.
A solenoid is not opening up enough and thus causing the above problem
I cleaned the filter with this ozone friendly spray, only to get very cold fingers.

To get her going again i just switch it over to petrol for 2ks then wait for the bowl to empty, and then back onto gas, 8/10 this will be ok. Otherwise run petrol till I get to work around 7ks.

Its fitted onto a Holden shuttle van 4cy with carby I think 2.2 or 2.6 1986

Any thoughts welcome.
P.S There is NO LPG gas service centre here in Why alla, there are back yard butchers, these will be my last choice.