I've had that problem quite frequently with my bass and each time it was a broken connection from inside the guitar. Be it a wire from the input to a volume/tone knob or to the pickup or what, but one of those little silver wires wasn't touching it all the time. How closely did you look at it, last time I looked in my bass for that problem I almost missed it as it naturally set in a way that made it look like it was fine and only lost connection when my bass was bumped. That's my best guess.