From the description, it seems it's a connection that is easily disturbed mechanically. That could be a bad solder joint (covered with solder, looks good, but solder is touching the wire instead of wetting it) as has been said. We say that all the time because it's most often the problem. But other possibilities are poor switch or socket connections, for example.
Does the problem occur with all three inputs? That's a test of the selector switch.
Another one that has come up recently on the forum somewhere is wires that get nicked when stripping, then break later right at the end of the insulation. It's very hard to see, and the insulation holds the parts close together so that slight mechanical shocks can make or break the connection. Usually wiggling each wire one at a time will make this one obvious ...
Another one is a mechanical connection that was not actually soldered.
