How can I fix my miele KF7642sn fridge freezer?

The refrigerator part of my Miele KF7642SN fridge freezer which is about 7 years old suddenly stopped automatically deforosting itself. Ice forms on the back wall above and within in the drainage channel thus preventing water from running away. Water then runs down over the ice and into the salad containers and onto the floor of the fridge below. The freezer section is OK the auto de-frost works fine. Lately the temeratures of both refrigerator and freezer have been increasing and decreasing of their own accord. This might be linked to the control board ?? Please advise how I can rectify the problem/s or is it too old being 7 years old and not worth repairing.