I think a bad battery is possible-seems like if it has a problem internally it won't get fully charged,and thus your alternator's going to keep working overtime trying to get it up to a full charge.Of course that'd be an amperage issue more than a voltage issue. My GMC's doing kinda the same thing,and I still haven't got IT sorted out. I think in my case it's just a byproduct of 50 years of "ranch-wiring".I do agree that weak grounds account for a lot of electrical problems,so that's a good place to start.
Speed
