Why does my charging system light go on and off while driving?

I have a 93 ford taurus 3.8L V6 automatic GL. The other day my charging system light started coming on in my car. It will come on and stay on for awhile and then go off. It has been doing this for two days straight. I have a battery/alternator tester that plugs into my cigarette lighter. The battery and alternator test find when the charging system light is off but when it comes on the alternator tests bad. Does this mean the alternator is going bad or what? Why is the charging light coming on and going off over and over? My alternator is about 2 1/2 years old and my battery is six months old.