How can I fix my Potterton EP2000 programmer?

The CH and HW letters keep flicking on and off when the heating comes on which is turning the bolier on and off! I turned it off at the wall (where the fuse is) and when I turned it back on and set the clock it worked ok for about 2 hours and then started doing it again!! I have no idea what to do and have no heating or hot water!

Many thanks