They all do this sir. Yes, even the brake lights. The fibre washer dishes until eventually the contact doesn't meet. Ultimately the contact will push right through. I fixed mine by supporting the fibre washer with a steel washer to stop it dishing. Cut a slot in the steel washer so it will go over the wire and add some suitable insulating material to stop it shorting. They have been OK ever since.
I thought about replacing the fittings but figured the new ones would do the same before long.
