Freezing indicates that there is a lack of calories. Does the freezing already start at the bottle, running up to the vaporizer? In that case the LPG bottle may be wrongly positioned with the dip tube up getting vapor instead of liquid gas to the vaporizer.
If the bottle is OK, it might be a lack of heat coming from the waterflow through the vaporizer regulator, with the engine running you should feel a regulator temperature of at least 50°C (122 F) If the heat is ok, it could be an defective primary stage vale that is leaking, letting past a quantity of liquid gas that is to important for the regulator to cope with. What make of LPG system do you have, Impco?

Scott
Thanks for all the info, it's very much appreciated and I will check all out????

Is the propane regulator freezing up or the vacuum fuel lock if the propane regulator is freezing up l would say first look at the thermostate make sure there there is a thermostate in the truck and it is opening.if that's OK then see if the water lines coming to the converter are cloged there should be water coming into the converter and back out into the block some time the block when gets age starts to scale. and could block the lines and that truck water lines to converter l believe are small about 5/16 next you could have a water pump that the fins are bad and rotted not pushing the water hope this helps.