Hi J & C;
The primary cause of this problem is operator error. This is what Impco says in a Service Bulletin they issued some years back. Starting an engine in extreme cold to move a vehicle a short distance, and stopping the engine without warming the coolant can result in trapping liquid propane in the heat exchanger. As this liquid vaporizes with the engine stopped, pressure in the heat exchanger will rise until excessive force is applied to closing the primary regulator valve against its seat. If this is occurring frequently the primary valve in the vaporizer regulator may become damaged.
Other items to look at are:
1. Water level of radiator
2. Vaporizer mounted higher than upper "radiator hose"
3. Defective radiator cap
4. Restriction in water lines (ck flow restrictor at reg.)
5. Water pump defective
6. Improper, missing or defective thermostat