my bet is a voltage regulator or alternator issue. Think about it. Your revving up, the serpentine belt is spinning faster, spinning the alternator faster, increasing output so I think at low revs you've got some degradation in the alternator brushings or the volt regulator is not keeping it at proper levels. I'm not an auto electric expert by any stretch, but I think this to be the most logical place, and may have something with the battery too