How can I get good picture on my Thomson freeview recorder?

My Thomson freeview recorder box i(DTI6300) is not working properly. The picture is rolling diagnally, its the same in menu, library and guide modes. It records but even programmes recorded before this problem occurred today are playing back in the same way (rolling picture). I have tried to reset by switching off and on again at the plug and have tried adjusting the aerial. We did have a lengthy power cut last night.