You went about this all the wrong way. You might have had low freon in your old unit, or it had a bad evaporator fan or fan wiring, or your door doesn't seal and air leaks in which could still be the problem. Make sure the machine is level so the door can seal correctly but check that seal too. Check the manual for a defrost cycle. If it has one (I never worked on that model of National machine) then yours might be set up wrong or you have a bad defrost element. Your cooling deck might also not be sealed correctly or it's installed wrong. Keep the machine away from the wall and make sure the vend screen underneath is kept clean.