07 ficus battery light intermittently comes on

The battery light on our 2007 Focus has been coming on intermittently for the last few days. I checked the charge on the battery, and it seemed fine. My first guess is the alternator. This problem is intermittent though, sometimes the car runs fine, other times it will die and need a jump start. After jump starting the car it runs fine for awhile. I know it seems like its just running until the battery is drained again, but sometimes it runs for longer than you think it would off just the battery. I still suspect the alternator though. I was thinking that the alternator is dying and just keeps trying to work properly, like its trying not to give up yet.
Any ideas fellas? Hope I'm not missing anything here. What the hell else could it be?