It might just need a new LPG filter, that would be my first move, and connect up a code reader which hopefully might point you to which cylinder isn't firing if it's definately missing on one, you could always remove the plug leads one by one and trace the miss the old way.

Might be a failing or simply a dirty solenoid not allowing the gas through on once cyl, assuming it's changing over and running ok its probably a simple cure.