2000 Jeep Cherokee Electrical Problems

On a recent road trip, the speakers started going out intermitently. We noticed that, when this happened the electrical odometer would also flicker and the voltage meter on the dash would swing between 13 - 15 volts. At night, we also noticed that the lights would flicker and dim.

As I said, this problem is intermitent and does not happen for hours. When it does, it seems to last not more than 15 - 20 mins.

Any thoughts?