Ford KA 1.3 Climate Zetec 2007?

Heater blows hot air. I have changed the valve under the wipers which did not help. I decided to change the control unit inside the car with the variable resister, this fixed the problem for a whole five minutes I moved the control from hot to cold and back and got the result I wanted and then it started blowing hot again and the control no longer had any effect. As the control I bought was second hand I thought maybe I was just unlucky and bit the bullet and bought another one with exactly the same results! At first it works fine then after a few minutes it packs up again. What on earth can this be? If there was a short surly it would blow straight away not after a few minutes. Anyone have any ideas? Thanks in advance. Pete