Moisture in the propane, freezing in the regulator is the correct answer. I had a new home built with propane heat. First winter when temps dropped below 10 degrees my heater would quit about 4:00 AM. Once the sun would come up and shortly before the repairman came, the heat would come back on. After about 3 trips for the repairman, he put some type of antifreeze or drygas in the tank, and that was the end of the problem. If your tank is portable, your propane supplier should be able to help you.