May be a dumb answer, but I've seen that happen with the socket. They get fickle and respond to wiggling and whacking. They get all corrody and give up on continuity.

2005 Model . Two Fast Eddy stickers , a bell and a clock . She's Lola . She tinkles and keeps time . http://s649.photobucket.com/albums/uu211/britbike05/