Mine is doing this too. She has been for a while. My alternator was replaced not too long ago and my battery *Diehard from 07* checks out OK. She also drops the voltage when I come to a stop or turn the wheel. It makes no sense. Nobody can figure it out. When I hit the brakes, it drops. When I throw her in neutral, it rises. Also, when you turn the wheel, the RPM'* are supposed to rise right? Well, mine drop. This doesn't make any sense to me. I've had her checked out by 3 or 4 people. Nobody can figure it out. I'm at the point where I'm saving money so I can replace her :(. I don't understand what it could be. Especially the whole braking thing. It makes no sense.
1999 Pontiac Bonneville SLE- Yvonne. Vonny for short. 205,000 on the dash
2005 Chevy Trailblazer LS EXT- 52K..bought with 13K on her in 08