What fault would cause the electrics to fail when switching off the socket?

The downstair sockets are on one ring which includes the kitchen appliances and lighting under the wall cupboards. An intermitant fault (once a week or so) has developed where the ring will trip at the fuse box when I switch off the socket controlling the under wall unit lighting. These lights have been installed for over 6 years, and the fault has only occured in the last 3 months. There is an audile bang and often a visible flash when it trips. Recently it has started doing it without touching the socket or activating anything in the house - ie when we are sitting watching TV?

Any thoughts most welcome?