I'm using a laptop playing MP3s and connecting it to a ministereo by using the headphone output of the laptop with a 25' cord plugged into the ministereo. Works OK except that it's started to hum pretty loud a lot of the time. Hum goes away if I jiggle the connection at the laptop enough, but comes back if it's jostled even a little. What's causing this and what can I do to stop it? I cleaned the plug with rubbing alcohol, but it hasn't solved this.