battery could be it so could instrument cords and patch cords and even speaker cords can do that. i once spent 30 min at practice wondering what was wrong only to find out that one of my speaker cords was only 1/2 way on the speaker input jack - problem solved.
anytime stuff like that happens take a deep breath and try to eliminate stuff one by one.
this is a bit gross, but time honored. put a fresh 9V batteries terminals on your tounge. ouch!. now take the battery out of your bass. and do the same thing. you'll notice if the bass' battery is signifcantly weaker.
