Check if both hoses to the regulator are warm. If not there may be a blockage at the intake manifold. One of our techs recently had a coolant hose blocked by a piece of gasket material possibly from a past waterpump or t-stat replacement. A frosted reg is almost always a coolant type issue.

check rubber hoses, sometimes they can swell inside restricting flow or kink where they make a bend but look OK on outside--check thermostat-might be stuck open or missing--if all that stuff is OK and its not the convertor--airlock is possible--fill system with coolant run with cap off till hot--install cap at first notch and run at half throttle for awwhile and remove slowly with rag on cap--this wiil burp any air

no coolant flow through the converter/regulator is a common problem with freezing up.
check for blockage in the lines, in the regulator itself, or in some rare cases the bypass circuit in the cooling system.
From my experience with Yale lifts its usually been a bad converter/regulator.
check the coolant level in the radiator

If you pull off the vapor supply line from the regulator to the carb and find gunk or coolant then thats a sign of bad things to come also.

Bad LP is not likely; LP fuel is not as clean as it used to be, you may find contamination inside the LP regulator which will affect the operation of the LP regulator and operation of the engine. Another possibility could be, the LP bottle might have a problem but that would be supply which would not cause the freeze up problem. I think you have a coolant supply problem to the LP regulator.

thanks for the list of things to check. I hope that helps me to take care of the problem. I wonder if we had a cylinder of bad gas if that would cause a similar problem? Just because it was fine for months and then started acting up out of the blue.

Chuck; an LP regulator is designed to be heated by using coolant from the engine. You must have coolant flow from the cooling system to the LP regulator. Any restriction in flow could cause the LP regulator to freeze. Some of the basic things to check would be.
1. Proper engine coontant level
2. Coolant flow from the water pump to the LP regulator back to the engine.
3. There might be an air lock in the coolant hose or the LP regulator.
4. There may also be an air lock inside the LP regulator.
5. In order to clear the system of this potenial air lock; slightly open the coolant hoses to and from the LP regulator until all the air is released. Be careful the fluid may be hot.
I may have missed a few items but this would be the basic's for this kind of problem. If you have an engine overheating problem along with the LP regulator freeze you may have a worn water pump or restricted radiator causing reduced flow to the engine and LP regulator.