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.