I would like to say it's the wall plate going bad, but that is very rare. I wonder if the wires to the contacts on the wall plate are loose, and you are actually moving the plate enough to make connection. I would pull the plate to see. The interior pipe is flex, so you can pull it out a ways to check it, but none of the connections are cemented, they are just slip fit. If you accidentally pull it apart inside, its clear the basement and take down the wall time