how can i fix my baxi 105e,temperature fault?

I have a baxi 105e which thinks it is at 70 degrees. The temperature lights are illuminated, even when cold. the heating only works when it is turned to past 70 degrees. The water works till it reaches 80 degrees then cuts out, only to restart when the temperature drops below 80. A new board and diaphram have been fitted. Any idea's.