I had the same problem, the evap thermostat is junk it will not open the contacts and the evap freezes up (chyna made junk). I solved mines by using a digital adjustable thermostat. I set the temp I want and the dead band and it cycles perfect. You can watch the evap get to target temp, relay drops out, temp rises, relay pulls in and no evap freeze up, here is a link to one similar to the one I used.
